Distributed Optical Combining: OBI Free, Power Free

ABSTRACT

Implementing a laser into a system that uses a remote powered combiner or an aggregating combiner (AC) with one or more in-range contributing combiners may provide power to the downstream contributing combiners (CC). The power may be provided on the same fiber that downstream signals and the upstream signals traverse.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)to U.S. Provisional Application No. 61/982,089 (ARR00207-P), filed onApr. 21, 2014, U.S. Provisional Application No. 62/043,793 (ARR00284-P),filed on Aug. 29, 2014, and U.S. Provisional Application No. 62/100,173(ARR00528-P), filed on Jan. 6, 2015.

BACKGROUND

An existing impairment of radio frequency over glass (RFoG)communication channels is Optical Beat Interference (OBI), whichafflicts traditional RFoG networks. OBI occurs when two or more reversepath transmitters are powered on, and are very close in wavelength toeach other. OBI limits upstream traffic, but also can limit downstreamtraffic. Existing efforts at mitigating OBI include adjusting ONUs to bewavelength specific, creating an RFoG-aware scheduler in the CMTS,changing ONU wavelengths in real-time, or combining multiple upstreaminputs nominally in the same wavelength range.

However, such solutions for reducing or eliminating OBI requireadditional power. For example, to combine multiple upstream inputsnominally in the same wavelength range without the occurrence of OBIrequires power to drive the photodiodes and the retransmitting laser.Some OBI reducing/eliminating devices have optical amplifiers, such asEDFAs, to aid in downstream splitting, which also requires additionalpower.

While distributed OBI free combining may result from employing afour-port combiner followed by four eight-port combiners and daisychaining them together, such solution requires power to be provided atfive spots (as opposed to power in just one spot with a thirty-two portcombiner). The increased need for power is in contrast to the concept ofFTTH, which is expected to be more passive and less reliant on powering.An exponential increase in power points is perceived as a reliabilityweakness in FTTH deployments, increases expenses, may not accommodatecurrently implemented splitting networks well, and may work well only ingreenfield applications where the power system is laid simultaneouslywith the fiber network.

Improved techniques for reducing distortions in a network without theexisting needs for increased power are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating embodiments described below, there areshown in the drawings example constructions of the embodiments; however,the embodiments are not limited to the specific methods andinstrumentalities disclosed. In the drawings:

FIG. 1 shows a system 100 for delivering content, e.g., CATV content, toa plurality of subscribers over a network, such as the RFoG networkdescribed above.

FIG. 2A depicts an optically pumped active combiner system, i.e., anopto-electronic system excited with light, or specifically populatingcertain electronic levels in a photo-detector to produce a photovoltaiccurrent, similar to how a photocell converts sunlight into electricalpower, as disclosed herein.

FIGS. 2B and 2C depict performance characteristics of a 980 nm pump.

FIG. 2D illustrates an amplifier, such as amplifier 133 in FIG. 1,converting high detector impedance to low laser impedance.

FIG. 2E depicts a bandwidth of impedance transforming a transistoramplifier driving laser with low impedance.

FIG. 2F depicts an example of contributing combiner 130, connected onthe left to an aggregating combiner 120.

FIG. 2G depicts an example EDFA in AC 120, where the WDM filter 605 atthe output of the EDFA is constructed such that 980 nm light ispermitted to leak to EDFA output 609 and this light is used to remotelypower a downstream CC 130.

FIG. 3 depicts a comparison of fiber length vs. the number of RFoGsplits for an RFoG architecture.

Referring to FIG. 4, in a traditional RFoG system 200 the CMTS 210 keepsthe RF level at a return input port constant.

FIG. 5 shows a system that mitigates laser clipping that might otherwiseresult from burst mode communications from an ONU.

FIG. 6 shows a second improved ONU that mitigates clipping.

FIG. 7 shows an ONU output spectrum having a rise time of 100 ns.

FIG. 8 shows an ONU output spectrum having a rise time of 1000 ns.

FIG. 9 shows a response time of an ONU to an RF signal.

FIG. 10 shows an ONU having a laser bias and RF amplifier gain control.

FIG. 11 shows the response time of an ONU with RF gain control inproportion to laser bias control.

FIG. 12 shows the response time of an ONU where the RF gain control isdelayed with respect to the laser bias control.

FIG. 13 shows an ONU having a separate amplifier gain and laser biascontrol.

FIG. 14 shows a transmission line receiver structure.

FIG. 15 shows a transmission line receiver connection to a biasedamplifier.

FIG. 16 shows a transmission line receiver with photocurrent detectionat the termination side.

FIG. 17 shows an active combiner with multiple inputs and optical burstmode operation.

FIG. 18 shows an active combiner with optical burst mode operationincluding amplifier bias control.

FIG. 19 shows an active combiner with OBM, laser bias, amplifier biasand gain control.

It is noted that while the accompanying Figures serve to illustrateembodiments of concepts that include the claimed invention, and explainvarious principles and advantages of those embodiments, the conceptsdisplayed are not necessary to understand the embodiments of the presentinvention, as the details depicted in the Figures would be readilyapparent to those of ordinary skill in the art having the benefit of thedescription herein.

DETAILED DESCRIPTION

Disclosed herein are enabling distributed OBI Free combining without theincreased need for power required by existing solutions. Embodimentsdisclosed for distributed OBI Free combining include systems that mayfunction without requiring a prohibitively larger number of powerlocations in the network than otherwise required in a network thatemploys OBI reduction techniques.

Modern CATV transmission systems have replaced much of the legacy RFtransmission path with a more effective optical network, creating ahybrid transmission system where cable content originates and terminatesas RF signals over coaxial cables, but is converted to optical signalsfor transmission over the bulk of the intervening distance between thecontent provider and the subscriber. Specifically, CATV networks includea head end at the content provider for receiving RF signals representingmany channels of content. The head end receives the respective RFcontent signals, multiplexes them using an RF combining network,converts the combined RF signal to an optical signal (typically by usingthe RF signal to modulate a laser) and outputs the optical signal to afiber-optic network that communicates the signal to one or more nodes,each proximate a group of subscribers. The node then reverses theconversion process by de-multiplexing the received optical signal andconverting it back to an RF signal so that it can be received byviewers.

Improvements to CATV architectures that provide further improvements indelivery of content include Fiber-to-the Premises (FTTP) architecturesthat replace the coaxial network between a node and a subscriber's homewith a fiber-optic network. Such architectures are also called RadioFrequency over Glass (RFoG) architectures. A benefit of RFoG is that itprovides for faster connection speeds and more bandwidth than currentcoaxial transmission paths are capable of delivering. For example, asingle copper coaxial pair conductor can carry six phone calls, while asingle fiber pair can carry more than 2.5 million phone callssimultaneously. FTTP also allows consumers to bundle theircommunications services to receive telephone, video, audio, television,any other digital data products or services simultaneously.

An RFoG topology includes an all-fiber service from the headend to afield node or optical network unit (ONU), which is typically located ator near the user's premises. In the headend, a downstream laser sends abroadcast signal that is optically split multiple times. The opticalnetwork unit, or ONU, recovers the RF broadcast signal and passes itinto the subscriber's coax network.

FIG. 1 shows a system 100 for delivering content, e.g., CATV content, toa plurality of subscribers over a network, such as the RFoG networkdescribed above. In example RFoG systems, the head end 110 deliverscontent to an ONU 140 at a customer's premises through a node. Thesystem architecture in FIG. 1 is shown with a head end 110 having atransmitter 112 that outputs a signal to a fiber link 118 of L1 km fordelivering a downstream signal to one or more splitters. The headendalso may include a receiver 114 connected to a WDM splitter 116 that mayreceive a signal from a fiber link 118 of L1 km. The fiber link 118 isconnected to an active splitter/combiner unit 120.

The example splitter/combiner unit 120 shown may include a WDM 122 thatseparates forward path signals from reverse path signals. As usedherein, the terms “forward path” and “downstream” may be interchangeablyused to refer to a path from a head end to a node, a node to anend-user, or a head end to an end user. Conversely, the terms “returnpath”, “reverse path” and “upstream” may be interchangeably used torefer to a path from an end user to a node, a node to a head end, or anend user to a head end.

The forward path signal from the WDM 122 is provided to a higher powermulti-port amplifier that distributes power. For example, FIG. 1 depictsa higher power multi-port amplifier that outputs an amplified opticalsignal to the active 1×32 splitter 126 that has 32 output ports torespective second fiber links 128. In embodiments, the higher powermulti-port amplifier is an Erbium Doped Fiber Amplifier (EDFA) 124 thatinternally distributes power over the 32 outputs of the combiner 120,each output operated at a relatively high power, e.g. approximately 18decibel-milliwatts (dBm). The WDM 122 may pass 1550 nm light from theEDFA 124 in the forward direction and direct reverser light, such as at1610 nm or 1310 nm, in the reverse direction to the receiver in theheadend 110.

At each port, the power level may be modest, e.g., in the 0-10 dBmrange. The active splitter/combiner unit 120 may be located within anode and a plurality of active/splitter combiner units 130 may beconnected to active splitter/combiner unit 120 over a short distance,such as 1-3 km or less through fiber 128. Each of the 32 ports of thesplitter/combiner 126 output, through a respective fiber 128, arespective signal to a second active splitter/combiner unit 130 of thesame type and configuration as the splitter/combiner unit 120. Anexample of an active splitter/combiner unit is ARIUS's AgileMax®©splitter/combiner unit. The length(s) of the fiber 128 may vary withrespect to each other. The output power per splitter 130 port is low,around 0 dBm. The splitter ports are connected to ONUS, for instance ina Multiple Dwelling Unit (MDU) or a neighborhood, via fiber 132 oflength L3.

In the reverse direction, either or both of the 1×32 port splitters 126,134 may operate as an active combiner 126, 134. Each combiner 126, 134may include (not shown) a WDM per port directing upstream light to adetector that converts the received optical signal to an electricalsignal and amplifies it in the RF domain. The combiner 126, 134 may thenprovide the electrical signal to a transmitter 129, 136 that outputslight at, for example, 1610 nm, 1310 nm, or some other appropriatewavelength, provided to the WDM 122 or 170, which in turn directs theupstream light into fiber, such as fiber 128 or fiber 118. Thus, alongthe return path from the subscriber's ONU 14 to the head end, asplitter/combiner 130 may operate to combine signals in the reversedirection for upstream transmission along fiber length 128, and asplitter/combiner 120 may operate to combine signals in the reversedirection for upstream transmission along fiber length 118. The combinedsignals may be transmitted upstream to a Cable Model Termination Service(CMTS) in the head end 110. Combined with the forward power limit on thefiber, the combined signals may require one forward fiber (L1 km) pergroup of 32 subscribers. At the head end, the fiber 118 is connected toWDM 116 that directs the upstream light to the receiver 114.

Optical networking units (ONUs) 140 in an RFoG environment terminate thefiber connection at a subscriber-side interface and convert traffic fordelivery over the in-home network at the customer premises. Coaxialcable can be used to connect the ONUs of an RFoG network to one or moreuser device, wherein the RFoG user devices can include cable modems,EMTAs, or set-top boxes, as with the user devices of an HFC network. Forexample, the R-ONU may connect to set-top boxes, cable modems, or likenetwork elements via coaxial cable, and one or more of the cable modemsmay connect to the subscriber's internal telephone wiring and/or topersonal computers or like devices via Ethernet or Wi-Fi connections.

The ONUs 140 convert the forward transmitted light to RF signals for thein-home coaxial network. The ONUs 140 also receive RF signals from thein-home network and modulate these signals onto a laser, operating at1610 nm for example, and the laser's output is sent upstream into thefiber L3. The upstream signal is combined with other upstream signals inthe combiner 134 and/or combiner 126 and transmitted further upstream infibers L2 and L1. At the WDM 122 the upstream signals are directedtowards the head end receivers 114 over fiber L1.

The sum of the fiber lengths L1+L2+L3 is up to 25 km. The system 100,however, will permit a higher total length of fiber between the head endand the ONUs, such as 40 km, because the system 100 can tolerate ahigher SNR loss, as further described below.

The upstream signals from the ONU 140 are individually terminateddirectly at the active splitter/combiner unit 130; even for ONUsoperating at 0 dBm, the power reaching the detectors is around −2 dBm(the fiber 132 is a short fiber up to a few km, and the WDM loss insidethe active combiner is small). This is almost 20 dB higher than inexisting RFoG systems, meaning that the RF levels after the detector inthe splitter 134 is almost 40 dB higher than in existing RFoG systems.As a consequence, the receiver noise figure is not critical and highbandwidth receivers can be used with relatively poor noise performance.The received RF signal is re-transmitted via the transmitter 136 alongthe reverse path into fiber 128 and received and retransmitted by thepreceding active splitter/combiner unit 120 and thereafter to the headend 110. Although the repeated re-transmission leads to some incrementalreduction in SNR, improvements in SNR from the active architectureprovides much greater overall performance relative to traditional RFoGsystems. More importantly, because all reverse signals are individuallyterminated at separate detectors, such as a multiple detector receiveras described in patent application (TBD), there can be no optical beatinterference (OBI) between different reverse signals. The reversesignals are not combined optically, hence OBI cannot occur.

In the forward direction there may be multiple EDFAs 124; these EDFAsare cost effective single stage devices with low powerdissipation—typically 2 Watts or less. Cascading the EDFAs results in anaccumulation of noise due to the finite noise figures of the EDFAs.Whereas the active splitter architecture does not require the EDFAs (thehigh power head end 110 EDFA could still be used to provide power to theONUs 140) the use of EDFAs inside the active splitter units may providesome advantages. For example, the complexity and power dissipation ofequipment in the head end 110 is greatly reduced, as is the fiber countemanating from the head end 110. The amount of power delivered to theONUs 140 is readily increased to approximately 0 dBm. As a consequence,ONU receivers obtain 12 dB more RF level from their detectors and do notneed as high an SNR or gain. Even with relaxed SNR requirements at theONU receivers, permitting a higher thermal noise contribution of thereceivers, the SNR impact due to EDFA noise is easily overcome due tothe higher received power. In addition, additional spectrum can besupported in the forward direction with an acceptable SNR relative tocurrent architectures, such as 4 GHz instead of 1 GHz in current RFoG,hence total data throughput rates can grow significantly without achange in operation to permit for example, services that provide 40 Gbpsdownload speeds and 10 Gbps upload speeds.

In embodiments, the optical combiner(s) 120, 130 provides upstream anddownstream RFoG capability and a completely transparent and reciprocalavenue for PON transmission. The optical combiner(s) 120, 130 may enablecomplete transparency for PON deployments. For example, the opticalcombiner(s) 120, 130 may enable OBI free and high capacity features bydeployment in compatible HFC D3.1 capable FTTH networks. Likewise, theoptical combiner 120, 130 may be incorporated in to GPON, 1G-EPON,XGPON1, 10G/1G-EPON, 10G/10G-EPON. The compatibility with HFC and D3.1enables the disclosed optical combiner(s) 120, 130 to be deployedalongside a current HFC network, and is D3.1 ready. The opticalcombiner(s) 120, 130 may be deployed on a fiber node, on multipledwelling units (MDU) and on single family home (SFU) deployments.

The optical combiner 120, 130 may be independent of ONUs, Cable Modemsand CMTSs. The optical combiner 120, 130 may be CMTS agnostic, thusrelieving the burden of creating an RFoG aware scheduler that is bothrestrictive and time consuming. The optical combiner 120, 130 assists tomake a cable version of FTTH more feasible, as compared to the PONalternatives. For example, in embodiments, the optical combiner 120, 130has a reciprocal PON Pass thru capability of the optical combiner 120,130 along with a high upstream and downstream capacity, which assistsRFoG deployment without interruption to the underlaid system orimpairment to future inclusion of PON functionality, such as later PONdeployment on an RFOG system.

Traditional RFoG and PON networks have a fixed power budget. This meansthat a longer reach results in lesser splits and a larger split resultsin shorter reach. This reach/split combination is a fundamental limit ofthese networks. The disclosed embodiments may enable both a longer reachand a larger Split. Thus, embodiments are described that may advanceFTTH topology and make FTTH deployment feasible.

In embodiments, the optical combiner 120, 130 has 32 ports, but onlyrequires one transmit port, one receive port, and one WDM component atthe headend 110. Thus, instead of requiring 32 WDMs and 32 receiveports, the optical combiner may save on head end space and power. Thecombiner 120 may be an active device that needs approximately 2 Watts ofpower. The optical combiner 120 may be powered by power sources readilyavailable in the RFoG system, or power can be provisioned in to theoptical combiner. The power source may include a battery back-up orsolar/fiber power alternatives. If the power is lost and the battery hasalso drained, the entire reciprocal PON transmission is unaffected. Theupstream RFoG transmission is however stopped. In a conventional RFoGsystem it would have been stopped also because the preponderance of OBIwould have severely impaired the system anyway if the system was atraditional RFoG system with a passive combiner. Also in case of a powerloss ONU (Optical Networking Unit) at the homes would cease to functionsuch that without any power backup such systems will cease to function,whether those are RFoG or PON systems, with or without the activecombiner disclosed here. The headend optical receiver may only need aninput power range from 0 . . . −3 dBm, and require 15 dB less RF outputpower due to the absence of the RF combiner such that with such a highoptical input power and low RF output power requirement the gain can below.

Embodiments for an RFoG combiner include preventing or eliminating OBIat the combiner as opposed to managing it at the extremities of thenetwork (such as using a CMTS scheduler at the headend side of thenetwork or wavelength specific ONUs at the subscriber end of thenetwork). Embodiments are described that enable elimination of OBI. Thedisclosed optical combiner may be used to eliminate OBI, enhancecapacity, and/or enable multiple services in RFoG, the cable version ofFTTH networks. The optical combiners 120, 130 may eliminate OBI, makingan OBI-free forever system. The optical combiner 120, 130 may beONU-independent, CM-independent, and CMTS-independent. The opticalcombiners 120, 130 enable long reach and large splits, e.g., up to 40 kmand 1024 Splits, which will expand even further. The high upstream anddownstream capacity enabled by the optical combiner 120, 130 includes upto 10G DS/1G US, and as high as 40G DS/10G US.

While distributed OBI free combining may result from employing afour-port combiner followed by four eight-port combiners and daisychaining them together, such solution requires power to be provided atfive spots (as opposed to power in just one spot with a thirty-two portcombiner). The increased need for power is in contrast to the concept ofFTTH, which is expected to be more passive and less reliant on powering.An exponential increase in power points is perceived as a reliabilityweakness in FTTH deployments, increases expenses, may not accommodatecurrently implemented splitting networks well, and may work well only ingreenfield applications where the power system is laid simultaneouslywith the fiber network.

Disclosed herein are enabling distributed OBI Free combining without theincreased need for power required by existing solutions. Disclosedherein are techniques for enabling active combiners to behave as passivecombiners, either combiner 120 or combiner 130.

EDFAs such as those in combiners 120, 130 are pumped by an emissionwavelength of a laser, conventionally pumped at 1480 nm which is theemission wavelength of a commercially available laser. Disclosed hereinare techniques for an OBI free combiner system that pumps the EDFA at awavelength that causes an active combiner to behave passively.

FIG. 2A depicts an optically pumped active combiner system, i.e., anopto-electronic system excited with light, or specifically populatingcertain electronic levels in a photo-detector to produce a photovoltaiccurrent, similar to how a photocell converts sunlight into electricalpower, as disclosed herein. Optical pumping conventionally means toinject light in order to electronically excite the transmission mediumor some of its constituents into other energy levels. In the context oflasers or laser amplifiers, optical amplification may be accomplishedvia stimulated emission for some range of optical frequencies, the widthof that range also referred to as the gain bandwidth. Here we also usethe term “optical pumping” to describe a system where a “pump” lightsource induces a photo current through the photo voltaic effect in adetector and the output voltage and current from the detector are usedto power electrical circuitry. For example, detector 137 may receivelight from a fiber 128 that was injected into the fiber with a “pump”laser at upstream location 120. Detector 128 may put out electricalpower to powering circuit 129 that performs necessary voltageconversions to provide power to electrical amplifier 133 for amplifyinginput signals. Thus with the means of a detector and a powering circuitamplifier 133 may use an optical pump for amplifying electrical inputsignals.

FIG. 2A depicts upstream signaling between combiners, such as combiners120 and 130 from FIG. 1. An OBI free combiner, such as the 32 portcombiner 120 described in FIG. 1, may be located about 20 km away fromthe hub, and is powered. The combiner 120 processes upstream signalsrepresented by block 201 (one of skill in the art would recognize thevarious components that may be used in combiner 120 for upstream signalsprocessing, generalized via block 201). Location 130 as shown in FIG. 2Aonly illustrates the upstream components (in FIG. 1, both upstream anddownstream components are shown, e.g., combiner/splitter 134, anamplifier 133 driving laser 136, a circuit that detects input power 138,a power management circuit 139 optionally with a power storage device135 and fed by a photo-detector 137 that provides energy to the powermanagement circuit).

FIG. 2A represents an upstream architecture for receiving signals froman optical network unit (ONU), e.g., where the ONU would be to the leftof (i.e., located downstream to) combiner 130. The combiner 130 includesan RF detector 134 for detecting whether an RF signal is present at acombiner input or alternatively a circuit 134 for detecting whetheroptical power is present at least one of the input fibers 132. Inembodiments, as shown in FIG. 2A, input fibers 132 may provide signalsfrom multiple ONUs to a multiple detector receiver 134. The multipledetector receiver(s) 134 may provide RF signals to an amplifier 133 thatdrives a laser 136. Thus, if a signal is detected by an RF detector 134,the RF detector 134 passes the signal through to an amplifier 133. Theamplifier 133 amplifies the RF signal including media content that ispassed through from the RF detector circuit. The amplified signal drivesthe laser 136. The laser's output is propagated to combiner 120 on afiber 128. The laser output is provided via an optical fiber 128(typically less than 1 km) to an upstream location 120 that furtherprocesses the signals.

As described in more detail below, while FIG. 2A depicts an activecircuit, such contributing combiners 130 may be remotely located at theedges of the RFoG network and not have access to direct power. Forexample, the contributing combiner 130 may not have access to electricalpower or may need to function in the absence of electrical power. TheOBI free combiner 130, located a distance from the hub, may includecomponents that need power to operate, such as an RF amplifier 133 todrive a laser 136 for upstream communication. As disclosed, an upstreamlocation(s) (i.e., upstream from contributing combiner 130) may includea pump laser to drive power to location 130.

As shown in FIG. 2A, the combiner 120 may include a pump laser 203. Thepump laser 203 at the upstream location (e.g., in combiner 130) mayinject light into fiber 128. The light output from the pump laser 203may be coupled to fiber 128 that also carries signals downstream fromthe combiner 120 to combiner 130 (as described above). The light outputfrom the pump may travel over the fiber 128 and is coupled from thefiber 128 to a detector 137 at the combiner 130. The power output fromthe detector is provided to a powering circuit 139 that powers the inputdetectors 134, amplifiers 133 and laser 136.

In this context, the powering circuit 139 may employ photovoltaics onthe light output from pump laser 203 to convert the optical energy ofthe light in to direct current electricity, much like a solar cellconverts light into electrical power. Thus, in contrast from the opticalpumping used for optical amplification in an EDFA, the disclosed fiberpower solutions use pump laser light and converts it with thephotovoltaic effect (like a PV cell) to electrical power. Thus, while apump laser puts out optical power, it should be understood that laser203 may be used as just a laser with a high output power. Such a lasertype is commonly used as a “pump laser” in EDFAs hence our reference tothe laser 203 as a pump laser. The remote powering via the pump laserdisclosed herein is particularly useful where a high amount of powermust be delivered to a detector, such as detector 137.

The disclosed techniques for remote powering enable active circuits tobehave passively. Remote powered components include components without alocal electrical connection for receiving power. For example, activecombiner 130 may not have a local electrical connection for power.

Shown in FIG. 2A are the upstream signal combining elements for purposesof illustration, and for simplicity, the downstream architecture is notshown in FIG. 2A. However, the architecture shown in FIG. 2A may includeelements for downstream signal distribution as described in FIG. 1.Thus, with reference to the downstream elements shown in FIG. 1 and FIG.2F below, a pump laser 203 in the active combiner 120 outputs from thefirst active combiner 120 to one or more downstream location(s) whichinclude a remote powered optical combiner 130, where the first activecombiner 120 is an aggregating combiner that may transmit to a pluralityof remotely-powered contributing active combiners 130. For example, asshown in more detail below with respect to FIG. 2F, an EDFA 510 may beremotely pumped for enabling downstream signal amplification.

FIGS. 2B and 2C depict performance characteristics of a 980 nm pump. Asdescribed above, a pump source including a laser in combiner 120 (e.g.,pump laser 203) may provide the pump energy to combiner 130 including RFamplifier 133 at a nominal wavelength of 980 nm to result in anamplified RF signal to drive a laser 136. As shown in more detail inFIG. 2F, a WDM 502 may separate the pump and signal wavelengths.

FIG. 2F further shows options for optically pumping an EDFA 510 usingthe pump light that will be explained further below. The pump sourcesuch as the pump including a laser 203 shown in FIG. 2A may also providethe pump energy to the EDFA 510 in FIG. 2F at a nominal wavelength of980 nm to result in an amplified downstream signal. As used herein, apump such as pump 203 is a laser for optical amplification. A pump maybe used in an erbium-doped fiber amplifier, as described in embodimentsherein. Thus, an incoming downstream signal, often at the nominalcarrier wavelength of 1550 nm, may be amplified by an EDFA 510.

In embodiments, a pump laser is located at downstream from thecontributing combiner and provides optical power over fiber 501. Forexample, a pump laser may be located at an ONU location that inputslight in to the contributing combiner 130 via fiber 501. For systemswith a separate fiber, such as fiber 501 shown in FIG. 2F, the fiber maybe a location located upstream or downstream from the contributingcombiner 130 that has an electrical power connection and, thus, is ableto provide sufficient optical power to the contributing power forproviding remote power.

FIG. 2B depicts the fiber loss across the wavelength spectrum for astandard single mode fiber (with a water peak). FIG. 2B shows the fiberattenuation in dB/km as a function of wavelength. As shown, a 980 nmwavelength has approximately 1 db loss/1 km of fiber (approximately 20%for each km). Over distances of approximately 20 km, such as a distancecommon between the headend and first optical combiner 120, is a 20 dBloss.

FIG. 2C is a curve fitting of FIG. 2B, but one where the water peakinduced loss has been eliminated (this is the case with the newer singlemode fibers). FIG. 2C shows a fit to the fiber attenuation as a functionof wavelength. The approximate 1 km distance between combiners 120 and130, therefore, means 1 dB loss, which means a loss of approx. 20% oflight. At distances of greater than 3 km of fiber, a corresponding 3 dbof loss means 50% of photons are not reaching detection, and the lossover 20 km of fiber using a 980 nm pump would be a significantimpairment to the system. Thus, 980 nm pumps are not used inconventional CATV networks for remote fiber powering solutions such asthe RFoG networks because of the excessive loss over transmissiondistances in such RFoG networks. While an EDFA may employ a pump for itsamplification purposes, it is noted that this is distinct from the useof the pump laser, e.g., 980 nm pump, as disclosed herein for remotepowering purposes.

Referring back to FIG. 2A, in embodiments of the disclosed techniquesfor optical combining, at least one aggregating combiner 120 isconnected to one or more contributing combiners 130 at a proximity forenabling the contributing combiners to behave passively. The active andpassive classification is based on whether the combiner configurationincludes active electronics (hence have overall gain at RF frequencies),or purely passive components (exhibiting an RF receive loss). Activecombiners contain active devices, such as amplifiers, that provide gain.Passive combiners contain no active or nonlinear elements, such astransistors and cannot produce more RF output power than is input to thecombiner because they are not provided with remote power for activeelements. Generally, passive components are those that cannot supplyenergy themselves, whereas an active components acts as a source ofenergy. Active components rely on a source of energy and usually caninject power into a circuit. Passive components can't introduce netenergy into the circuit. They also can't rely on a source of power,except for what is available from the (AC) circuit they are connectedto. As a consequence they can't amplify (increase the power of asignal).

By behaving passively, the disclosed active combiner 120 is still anactive combiner but enabled via remote powering vs. being truly passive.In other words, combiner 130 does include transistors and does have RFgain, but it does not require an electrical power connection because thepower required to run it is provided optically via fiber in accordancewith the disclosed techniques. Thus, for an installer and operator ofthe system for all practical purposes, the combiner 130 appears andbehaves as if it was a passive combiner, though it is still an activecombiner where the electrical power is provided by a photo-detectorilluminated by a powerful optical source delivered to the detector viafiber.

In an example implementation, a 4 . . . 32 (between 4 and 32 port) portOBI free aggregating combiner (AC) 120 is connected to four or more ofeight port contributing combiners (CC) 130 downstream. Disclosed hereinare techniques for enabling the contributing combiners with their ownpower, such that the contributing combiners behave passively in thesense that they do not need a local electrical power connection. Suchaggregating combiner and contributing combiners are co-located at alocation remote from the hub, where the distance between aggregatingcombiner 120 and the one or more contributing combiners 130 may bearound 1-2 km, whereas over a 1-2 km distance (e.g., the distancebetween combiners 120 and 130) the loss using a 980 nm pump isapproximately 1 dB.

As disclosed herein for techniques for optimizing transmissions over thedistances between the headend, the combiners, and the customer premises,980 nm pumps are implemented with the combiner 130 in embodiments topower the downstream contributing combiners (CC) on the same fiber thatdownstream signals and the upstream signals traverse. The 980 nm pumpused as a pump laser 203 may be the same as a pump used in the EDFAs(e.g., FIG. 1, EDFA 124) at the 4 . . . 32 port aggregating combiner(AC) 120 location, but with the distinct purpose as disclosed herein forremote powering the contributing combiner. In other words, the 980 nmpump may extend power to a location that would not otherwise have power,i.e., enabling remote powering of active combiners 130. The pump used inthe combiner 120 does not use a pump to produce electrical energy forits own circuitry since it already has an electrical power connection.In some cases, combiner 120 could be receiving remote optical power;however, in such cases combiner 120 would not contain the 980 pumpsources to power 130 because they take too much energy, i.e., the pumpsrequire too much energy to drive them off a remote fiber power link;instead they are used to provide power to remote fiber power links.

In this manner, the active combiners 130 may behave passively meaningthe active combiner does not require a local electrical powerconnection, i.e., the active combiners may be remote powered. Suchremote powered combiners may be ideal for reducing power requirementswhere there may be many more combiners 130 in a system than combiners120 (e.g., for each aggregating combiner 120 there may be 32contributing combiners 130). Further, remote powering enables an activecombiner without a local electrical connection providing power to bepowered, and enables the active combiner to behave passively withregards to power, i.e., receiving power remotely instead of activelyproducing or requiring it. Combiner 120 may likewise be made to behavepassively if the distance between the combiner 120 and the componentproviding optical power to combiner 120 enables use of the 980 nm pumpto provide power over fiber.

It is noted that the 980 nm pumps disclosed herein are similar to thelaser used to provide pump power for the EDFA, but, unlike the EDFApumps that require wavelength stabilization for efficient pumping, the980 nm pump in the combiner 130 need not be wavelength stabilized.Further, because the 980 nm pump may output a large amount of power(e.g., 30 dBM), it may be desirable to SBS mitigate the light. SBS is awell-known effect where optical fiber reflects light above a certainthreshold, methods to mitigate SBS include broadening of the spectrum ofthe light source. The distance over which the 980 nm pump is operatingfor is modest, (e.g., 1 km between combiners 120 and 130), but even ifmost light will be received over a modest fiber length, the 980 nm pumpin the remotely powered combiner 130 may be SBS suppressed/mitigated toenable even more of the light to be received.

It is to be noted here that in this application, while 980 nm wavelengthis described as the preferred fiber power for powering the contributingcombiners, other wavelengths such as 850 nm and 1480 nm may also beused. While there are less expensive sources or higher power sources at850 nm, the fiber loss is higher at the higher power wavelengthoffsetting some of the advantages. Furthermore, optical components, suchas WDMs for single mode operation are much more available for the 980 nmand the 1480 nm sources due to their use in current EDFAs. While the1480 nm pumps are available with single mode components and have thedistinct advantage that the fiber loss is a much more modest than thatat 980 nm—the loss at 1480 nm is around 0.25 dB/km compared to the 1dB/km at 980 nm—the 980 nm wavelength may provide advantages in respectof the fiber power generation by virtue of its lower wavelength andconsequent higher energy photons. Furthermore, the 1480 nm may alsolimit the use of 1490 nm downstream 1G PON wavelength application thuslimiting the PON transmission. In case the end user does not anticipatePON operation and reliable efficient fiber power is designed, then the1480 nm operation would provide longer reach, and all the advantagesdescribed for the 980 nm operation described above and to be describedbelow.

Nevertheless, it is to be understood here that while this applicationdescribes 980 nm pumping, other wavelengths notably the 850 nm and 1480nm may be used subject to the discussion above. It is also possible thatthe 1550 nm signaling transmitter could be co-located within the samemodule with either the 980 nm or the 1480 nm pumps. For example, inremote CCAP architectures, link length is often shorter between a headedand an aggregating combiner 130. Thus, the 1480 nm pump or the 980 nmpump may be located in the same transmitter module as a 1550 nmsignaling laser.

In embodiments for including a 980 nm pump, the 980 nm wavelength has a1 dB/km of loss. For distributed contributing combiners 130 locatedwithin 1 km of the AC (Active Combiner) 120, use of the 980 nm pumptherefore results in minimal losses. In embodiments, the 980 nmwavelengths are received by Si detectors, which have a sufficientband-gap to produce power for operating the CCs (ContributingCombiners). As described above, other wavelengths such as 850 nm and1480 nm may be used subject to the discussion presented.

Contributing combiners 130 may be an 8 port device followed by aretransmitting laser. Since these CCs are very close to the AC, thepower of the CC need not be very high. For example −3 dBm or −6 dBmwould suffice. Typical power required to run a laser at this power oreven up to 3 dBm is around 50 mW or so of power. An AC or a CC may havepassive photo diode coupling and therefore not require large power,e.g., approx. 5 mW of power to power photodetectors and 70 mW to drivethe RF amplifier in the CC. A typical economical SM 980 nm pump provides26 dBm of power; there is a 1 dB loss for a WDM to inject 980 nm lightonto the SMF and with a 1 dB loss it reaches the silica detector at 24dBm of power. With a 50% efficient detector, the released power is 21dBm, which is around 125 mW, sufficient to run the CC.

In embodiments, the 980 nm WL pump causes no optical non-linearinteraction between 980 nm and 1550 nm light in as short a distance as 1km.

In embodiments, the 980 nm pump is SBS suppressed to enable 22 dBm ofpower across the SMF. A typical SBS limit is 7 dBm at 20 km andapproximately 15 dBm-18 dBm at 1 km of fiber. Thus, in embodiments, amodest modulation on the 980 nm pump enables SBS suppression.

In embodiments, the powering circuit takes a signal from an input powerdetect circuit connected to the input detectors that flags if opticalpower is incident to the input detectors. This signal can then be usedthe control the power provided to the laser and/or the amplifier.Optionally a power storage device such as a capacitor, super-capacitoror battery is coupled to the powering circuit to support powering of thecombiner. Note the pump laser may be located at an upstream location asshown but may also be located at a downstream or other location toprovide power to the active combiner via an input fiber or separatefiber.

Whereas the figure above shows elements of a typical active combinerused in systems such as that described in FIG. 1 such combinersconventionally need several Watts to power the (already efficient)multiple detector receiver, amplifier and laser. Whereas the lasertypically requires just around 1 Volt and under 50 mA of current tooperate, equating to around 50 mW of power, the RF amplifiers consume alarge amount of power. One main reason is that RF amplifiers aretypically designed as gain blocks that operate in systems withcharacteristic impedance such as 50 or 75 Ohm. Photo detectors on theother hand are high impedance sources and ideally operate at highimpedance, limited only by the attainable characteristic impedance of atransmission line if operated with multiple detectors in a transmissionline structure. Lasers on the other hand are low impedance devicespresenting a load impedance of typically a few ohms to an amplifier.

When taking into account that the fiber length to the active combiner istypically small it becomes clear that the optical input power to theinput detectors is significant, typically 1-2 mW inducing 1.2 mA ofinput detector current that can be provided into an impedance up to afew 100 Ohm resulting in up to around 400 uW of signal power. The laserpeak modulation current is on the order of 20 mA with an impedance of afew Ohm or up to around 800 uW of signal power. It becomes clear thatnot much signal power gain is required to drive the laser; however animpedance transformation is required. Impedance transformation ispossible with transformers, however transformers induce losses insteadof providing a small amount of gain and it is difficult to obtain a veryhigh impedance transformation ration with a large RF bandwidth. In thisapplication preferably an RF bandwidth of 5-1000 MHz is obtained.

Note that the input detector structure, even if biased at 5 V, onlyconsumes around 10 mW when 2 mA detector current is induced, thus it canbe concluded that the RF amplifier is the greatest consumer of opticalpower. The discussion in the previous paragraph explains that atraditional RF amplifier with high gain and 50 or 75 Ohm input andoutput impedances is not ideal in this application. This can in part beamended with transformers but these induce additional loss and causebandwidth imitations. Therefore a different solution is sought after;here a transistor amplifier is primarily used as an impedancetransformer that also provides a limited amount of gain.

FIG. 2D illustrates an amplifier, such as amplifier 133 in FIG. 1,converting high detector impedance to low laser impedance. As shown inFIG. 2D, the transistor amplifier may be driven by a source with a highimpedance (such as R6 with an impedance of 150 Ohm) and photo-diodesrepresented by current source I1. The transistors have bias networks L3,L4, L1, L2, R4 to set up a bias current. The input and output impedanceof the transistor amplifier is set with feedback resistors R7, R8 andR3, R10. These are set such that the low laser impedance R1 of a few Ohmis converted to an impedance close to R6 at the input to the transistoramplifier. Note that due to the low laser impedance the signal voltageswing at the laser is very small; as a consequence a supply voltage tothe amplifier as low as 1 V is sufficient to operate the amplifier. Theamplifier current required is on the same order as the laser current(around 50 mA) to obtain acceptable distortion performance.

Semiconductor lasers operating in the 1310-1610 nm range can operatewith a voltage drop in the 0.9-1.5 Volt range; this means that thevoltage source required to operate the laser could also provide thepower required to operate such an amplifier. As a consequence the entirecircuit to operate photo-detectors, amplifier and semiconductor lasercould operate on one low current voltage source to bias the detectorsand one low voltage source to operate both the laser and the amplifier.In some cases the required detector bias could also be as low as 1 V(but not typically) so that a single low voltage supply may besufficient.

FIG. 2E depicts a bandwidth of impedance transforming a transistoramplifier driving laser with low impedance. As illustrated, bandwidthobtained is high, on the order of 1000 MHz.

In general the laser impedance at 1 GHz is affected by parasiticinductance of a laser package. This parasitic inductance affects theimpedance transformation by the transistor amplifier and can bemitigated by adding appropriate inductance to feedback resistance toresistors R3 and R10. This was included in the calculation shown above.

As described in more detail below, aspects of ONU designs with laseron/off control and RF gain on/off control apply to the combinersdescribed above, and can additionally be used to save power and preventturn-on and clipping related errors.

FIG. 2F depicts an example of contributing combiner 130, connected onthe left to an aggregating combiner 120. A fiber 500 provides downstreamsignals and pump laser power at 980 nm to an EDFA 510. This power may bepassed directly to the EDFA or coupled to a splitter 506 using WDMcomponent 502 that selectively couples all or part of the 980 nm pumplight to that splitter. Splitter 506 provides all or part of the powerinto splitter 506 to detector 507 that provides power to the upstreamcircuitry. Remaining power from splitter 506 is provided to WDMcomponent 503 that couples power to the Er doped fiber section 504 inEDFA 510. At the output of EDFA 520 a WDM component 505 couplesremaining 980 nm pump light to an optional detector 508 that can also beused to provide power to upstream circuitry and the WDM component 505 toprevent leakage of pump light from the EDFA to output 509. Each and anyof these implementations may be chosen provided that power is providedboth to the upstream circuitry and an EDFA in CC 130.

Note that FIG. 2F only illustrates 980 nm pump light distribution andnot other components in CC130 used to combine and split other opticalsignals. Also note that alternatively 1480 nm may be used to providepower to both EDFA and upstream circuitry. Finally note that in case1480 nm is used then wavelengths longer than 1480 nm, such as 1550 nmand 1610 nm can be amplified by the Raman gain effect on a fiberpreceding the CC. Preferably 1550 nm is used in the downstream and 1610in the upstream direction to and from the CC respectively. If a 1480 nmpump is used with sufficient power then the Raman gain effect on thefiber preceding the CC can provide enough gain (for instance greaterthan 1 dB) so that the Er doped fiber section of the CC is not required.

FIG. 2G depicts an example EDFA in AC 120, where the WDM filter 605 atthe output of the EDFA is constructed such that 980 nm light ispermitted to leak to EDFA output 609 and this light is used to remotelypower a downstream CC 130. The EDFA has a pump laser 606 that is coupledtowards the Er doped fiber 604 using WDM coupler 603. This means thatcoupler 605 that is conventionally present in an EDFA to rejectremaining 980 nm pump light may be absent in the EDFA. This figure onlyillustrates 980 nm light distribution and not the other components usedfor RF signal handling; the output fiber 500 from AC 120 to CC 130 willbe coupled to output 609 such that 980 nm is passed to fiber 500.

Described above, it is noted that while 980 nm may be used for fiberpower in the upstream, the 980 nm may also be used for remote pumpingthe downstream EDFA. For example, if the system uses an EDFA (e.g., EDFA510 in contributing combiner 130 shown in FIG. 2F), then the 980 nm mayenter that EDFA 510 directly through the input or even without an inputisolator to get to the Er-doped section. Then, at the end of theEr-doped section the remaining pump light can be dumped to the detectorthat powers the upstream. The 980 nm may be used for remote pumping thedownstream EDFA510 using just one fiber or multiple fibers. For example,one fiber may be used for sending upstream and downstream signals, asecond fiber may be used for fiber power to power upstream signalprocessing circuitry, and a third fiber for remote pump power to pump anEDFA for downstream signal amplification. Thus with 3 or more fibers, afully functional totally passively behaving contributing combiner havingDS amplification and US OBI Free operation is possible. Often when fiberis laid in the access extremity, additional strands are also laid andare readily available. However, the above use of the 980 nm pump mayalso be employed with a single fiber.

If only one fiber is utilized, a high power 980 nm pump in theaggregating combiner is WDMed into the signal fiber, e.g., fiber 128,that extends to one or more contributing combiners 130, connectingcombiners 120, 130 via the single fiber and thereby coupling the 980 nmpump to a receiver or a detector in the contributing combiner 130. Aportion of the power may then be sent to the Si or InGasP receiver andconverted to electrons. However the remaining portion is sent directlyto the passive EDFA and is used to amplify the downstream signal.Details of this are illustrated in FIGS. 2F and 2G.

Use of the 980 nm as disclosed herein enables a completely remotepowered operation for the DS and US operation of the contributingcombiner 130. The innovative combination of fiber power and remotepumping is unavailable in conventional systems that do not contemplatethe aggregating and contributing combiner concepts. Further, the optionto use either single fiber or multiple fiber operation depending uponthe pump power availability, cost, and fiber availability, is madeavailable using the disclosed techniques. As described above 1480 nm mayalso be used in case the end user does not intend to use 1490 nm PON.Under proper design, 1480 nm pumps may allow for a longer reach to thecontributing combiners.

In embodiments, the disclosed optical combiner prevents interference inRFOG deployments in the combiner rather than preventing interferenceusing measures taken in the ONU where previous attempts have failed orproven to be cost-prohibitive.

Traditional RFoG architectures have a fixed power budget. This meansthat as fiber length between the head end and the ONUs increases, asmaller number of splits may be used, as can be seen in FIG. 3.Conversely, the more splits that are desired, the less fiber length maybe deployed. The disclosed active architecture, however, enables fiberlength of up to approximately 40 km irrespective of the number of splitsused, meaning that the disclosed active architecture permits fiberlengths of 40 km or more along with a large number of splits, e.g. 1024,thereby advancing FTTP topology and deployment.

The overall cost of the active splitter architecture shown in FIG. 2 issimilar to that of a traditional RFoG solution. The cost of activesplitter EDFA gain blocks and WDM and detector components in the activearchitecture is offset by the elimination of head end gear such asreceivers, high power EDFAs and combiners. A cost reduction of the ONUsthat can operate with lower output power further supports the activesplitter architecture. Further advantages of the active splitterarchitecture may include a reduction in outgoing fiber count from thehead end, which can have a large impact on system cost, as well as anoption to use 1310 nm reverse ONUs while staying within a typical SNRloss budget, which can further reduce costs. Also, the system shown inFIG. 2 exhibits increased bandwidth relative to what existing RFOGarchitectures are capable of providing, avoiding limits on service groupsizes and concomitant requirements for more CMTS return ports. Finally,unlike OBI mitigation techniques in existing RFoG architectures, thesystem shown in FIG. 2 does not require cooled or temperature controlledoptics and bi-directional communication links that necessitateadditional ONU intelligence.

Each of these factors provides a further cost advantage of an activesplitter solution over existing RFoG architectures. Required space andpower in the head end is also reduced; the active splitter solutionrequires one transmit port, one receive port and one WDM component.Existing RFoG architectures, on the other hand, requires transmit ports,multi-port high power EDFAs, 32 WDM's, 32 receiver ports, and a 32-portRF combiner. Existing RFoG architectures require very low noise, highgain, and output power receivers with squelch methods implemented toovercome power loss and noise addition in the RF combiner. The system100 shown in FIG. 2, conversely, works with input power normally in the0-3 dBm range, little gain is required, and requires 15 dB less poweroutput due to the absence of the RF combiner before the CMTS.

The disclosed optical combiner unit may be independent of ONUs CableModems and CMTSs. The disclosed optical combiner may be CMTS agnostic,thus relieving the burden of creating an RFoG aware scheduler that isboth restrictive and time consuming. The optical combiner assists tomake a cable version of FTTH more feasible, as compared to the PassiveOptical Network (PON) alternatives. For example, the disclosed opticalcombiner unit may have a reciprocal PON pass-thru capability of theoptical combiner unit along with a high upstream and downstreamcapacity, which assists RFoG deployment without interruption to theunderlying system or later PON deployment on an RFoG system.

Preferably, the disclosed optical combiner unit implements atransmission line approach to combine multiple optical photodetectors ina single optical receiver. This may be accomplished in configurationswith unidirectional or bidirectional communication. A unidirectionalsystem provides no control communication signals from an active opticalsplitter to an ONU, i.e. control communication signals only pass from anONU to an active splitter. Thus, in a unidirectional system, an activeoptical splitter simply accepts an output level from an ONU and operateswith that output level. A bidirectional system passes control signalsfrom an active optical splitter to ONUs instructing them to adjust theiroutput power; this type of system permits accurate equalization of theinput levels to the active optical splitter from each ONU.

Some active splitter/combiner systems may preferably include redundancywhere active optical splitters switch their return laser power (thereturn laser that carries the combined information of the ONUs connectedto it) between a high and a low power state or operates this laser in CWmode. In that case an upstream head end or active optical splitter caneasily detect loss of power at an input port and enable a second inputport connected to another fiber route to receive the information, in theforward path the other fiber route would also be activated in this casebecause generally the forward and reverse light share the same fiber.Also, some active splitter/combiner systems may include a reverse laserin the active optical splitter that adjusts its power output as afunction of the number of ONUs transmitter to the active opticalsplitter and the photocurrent received from these ONUs. Still otheractive splitter/combiner systems may have a gain factor and reverselaser power of the active optical splitter set to a fixed value.

Preferably, the disclosed optical combiner unit is able to configureitself under changing circumstances. Instances occur in which cablemodems in the ONU are required to communicate with the CMTS even ifthere is no data to be transmitted. Usually, however, the ONU is turnedoff during periods when there is no data to be transmitted between theONU and CMTS, and a cable modem could go hours before receiving orsending data. Thus, in some embodiments the disclosed combiner unit maybe configured to stay in communication with the CMTS. Cable modems maybe required to communicate back to the CMTS once every 30 seconds, orsome other appropriate interval.

ONU Operational Modes and Laser Clipping Prevention

In traditional RFoG architectures, ONUs transmit information in burstsand at any point in time one or more ONUs can power on and begintransmitting information. As required by the DOCSIS specification, allONUs are polled repeatedly with an interval up to 5 minutes but usuallyless. When an ONU turns on, the optical power transmitted by the ONUrises from zero to the nominal output power in a short time. As aconsequence, the optical power received by the active splitter from thatONU goes through that same transition. The slew rate with which the ONUcan turn on is constrained by the DOCSIS specification, but thetransition is still relatively abrupt, resembling a step function. As iswell known from signal theory, a step function has a frequency spectrumthat contains significant energy in the low frequencies, with decliningenergy as frequency rises. If the low frequency energy were allowed tobe re-transmitted unimpeded by the active splitter laser whenretransmitting signals, then the signal could readily overdrive thelaser and cause laser clipping. To avoid such clipping, severalapproaches may be utilized.

First, a steep high pass filter may be implemented after the detectorsof the active splitter, which ensures that the low frequency signalsinduced in the photo detectors from ONUs that power on and off do notoverdrive the laser used for retransmission. Such a high pass filtershould be constructed such that it presents a low impedance to the photodetectors for low frequencies such that the photo detectors do not see asignificant bias fluctuation when ONUs turn on and off. For instance ifa coupling capacitor were used as the first element in a filter thatpresents a high impedance to the photo-detectors then an ONU that turnson could result in a significant bias fluctuation of the photodetectors, so such a type of filter should not be used. In this context,a significant bias fluctuation would be a fluctuation of greater than10%. Preferably, the high pass filter is configured to limitfluctuations to levels well below this figure, e.g., 5% or even 2%.Also, if the re-transmitting laser is also used in burst mode, then theslew rate of the retransmitting laser should preferably be limited whenit turns on, so as to limit the amount of low frequency spectrum intothe photo-detectors of preceding active splitter units.

As noted above, ONUs normally operate in burst mode and this causes theassociated problems just described. Burst mode operation of the ONUs isrequired in an existing RFoG architecture because otherwise, theprobability of OBI occurrence would be very high and the system wouldnot generally work. With the active splitter architecture, however, OBIcannot occur and the signal to noise margin is much higher than withRFoG. Because of this, a second approach to reducing clipping is tooperate ONUs in a continuous “on” state with the active architecturepreviously described. For 32 ONUs delivering signals into an activesplitter, the shot noise and laser noise accumulates, but the signal tonoise budget is so high that the resulting SNR performance is still muchbetter relative to existing RFoG systems. As a consequence, the activesplitter architecture allows operation of all connected ONUssimultaneously given that the active splitter architecture eliminatesOBI.

A third option to alleviate laser clipping is to allow the ONUs tooperate in burst mode, but to detect the amount of power out of the ONUand attenuate the ONU's signal so as to prevent clipping. Referring toFIG. 4, in a traditional RFoG system 200 the CMTS 210 keeps the RF levelat a return input port constant. The return signal is generated by acable modem 220, provided to an ONU 230 that includes an optical reversetransmitter and relayed over an optical network 240 to a receiver 250co-located with the CMTS that converts the optical signal back to an RFsignal and provides that to the CMTS 210. It should be understood thatthe optical network 240 can contain active and passive elements. Itshould also be understood that the communication between the cable modem220 and the CMTS 210 is bidirectional, i.e. there are both “forward” and“reverse” path signals.

The communication path shown in FIG. 4 is used to adjust the outputlevel of the cable modem. In case the loss from the ONU 230 to thereceiver 250 is high, or the loss from receiver 250 to the CMTS 210 ishigh, then the CMTS 210 will adjust the output level of the cable modem220 to a high level in order to obtain a set input level at the CMTS ora level within a predefined range at the CMTS. In traditional RFoGsystems there is considerable margin on the input level that the ONU canhandle, to allow for this adjustment. However, it is still possible forthe cable modem to overdrive the ONU 230, particularly as the amount ofspectrum used by the cable modem increases to support future heavy dataloads. When the ONU 230 is over-driven, then the RF signal modulatedonto the laser of the ONU 230 becomes so high that the reverse laser inthe ONU 230 is driven into clipping, i.e. the output power from thelaser swings so low that the laser is turned off. This causes severesignal distortions and creates a wide spectrum of frequencies thatinterferes with communication throughout that spectrum.

The optical network typically combines signals from multiple ONUs, eachONU is typically communicating in another band of the frequencyspectrum. The communication of all of these ONUs is affected by the widespectrum induced by the distortions even if only one ONU is clipping.Preferably this problem is resolved in such a way that the other ONUsare not affected, the clipping ONU is brought to a state where it canstill communicate and the CMTS produces a warning that an ONU is notoperating optimally.

A variation on the third option just described is to operate ONUs inburst mode where the ONU switches between a low power state (forinstance −6 dBm) and a high power state (for instance 0 dBm). This meansthat the ONU laser never fully turns off, i.e. the laser always operatesabove its laser threshold, and can always be monitored by the activesplitter. The reduction in output power when it is not transmitting RFsignals reduces the shot and laser noise accumulated in the activesplitter such that the signal to noise impact is minimized.

In circumstances where the optical combiner unit cycles to a low powerstate rather than a completely off state, the photodiode current and amax/min can be tracked for photodiode current across all of the ports ofthe combiner, and thus a microcontroller can be used at the opticalcombiner to continuously track the max and min in a specified timeinterval. For example, if for ten minutes the photodiode current max is0, then the optical combiner determines that the cable modem is eithernot connected, has a defective optical link, or is otherwise defective.Optionally the active optical combiner can signal absence ofphoto-current to a head end. The optical combiner is also able toconfigure itself whether or not the optical combiner can determine iflight received is received is bursty as in normal RFoG operation or CW(continuous wave) as with a node reverse transmitter. The opticalcombiner is able to know by using CMTS upstream signaling imposed by theCMTS onto the modems to analyze which ports are working, which ports aresilent, which input ports are connected to ONUs, and which input portsare connected to optical combiner reverse transmitters, where opticalcombiner ports may have an output power profile different from ONUs inthe sense that the power may be CW or may be fluctuating between a lowand a high power state or may carry information embedded in thesignaling indicating the presence of a further optical combiner betweenthe ONU and the optical combiner.

For cascaded active splitters, the return lasers in cascaded activesplitters can similarly be operated in conventional burst mode where thelaser turns off between bursts, in CW mode, or in a burst mode thatswitches between a high and a low power state. It should also beunderstood that CW operation of reverse lasers and/or ONUs, or burstmode operation with a low and a high level further facilitatesdetermination of the optical input levels into the upstream input portsof active splitters. It should also be understood that, although thedevices and methods disclosed in the present application that prevent orotherwise reduce clipping by a laser operating in burst mode wasdescribed in the context of an ONU, the devices and methods used toprevent clipping by a laser in an ONU are equally applicable topreventing clipping by a laser in an active splitter as previouslydisclosed.

FIG. 5 shows a system that mitigates laser clipping that might otherwiseresult from burst mode communications from an ONU. Specifically, an ONU300 may include an RF rms detector 310, a microcontroller 320 and analgorithm to adjust an attenuator 330 in the ONU as a result of thepower detected at the RF rms detector 310. The reverse path from the ONU300 may be operated in burst mode; when an RF signal is presented to theinput 340 then the ONU's laser 350 is turned on by the bias circuit 360.This can be accomplished either by an additional RF detector 315 in theinput circuit directly turning on the bias circuit (dashed arrow) or bythe RF detector 310 and the microcontroller 320 turning on the bias andsetting the bias level. When a burst occurs, the RF detector 310measures a power level and provides that to the microcontroller 320. Themicrocontroller also is aware of the operating current of the laser 350as set by the bias circuit 360. Thus, the microcontroller 320 cancompute if the RF signal level is large enough to induce clipping of thereverse laser. If no clipping will occur, no further action needs to betaken and the ONU 300 can retain a nominal RF attenuation value. If, atthat time, the ONU is not at a nominal RF attenuation value theprocedure is more complicated, this will be discussed later in thespecification.

If clipping will occur, the microcontroller 320 stores the event. If aspecified number of clipping events has been counted within a specifiedtime interval, then the microcontroller 320 determines that the ONU 300is having significant performance degradation due to clipping, and isalso significantly impairing other ONUs in the system. In that case, themicrocontroller 320 computes how much the RF attenuation needs to beincreased to eliminate the clipping using RF power measurements thathave been previously recorded. The microcontroller 320 then increasesthe RF attenuation to a new value such that the laser 350 is modulatedmore strongly than normal (more modulation index than the nominalvalue), but still below clipping. The microcontroller 320 may optionallyalso increase the laser bias setting to provide more headroom for lasermodulation.

Because attenuation of the signal from the ONU 300 has been increased,the RF level as seen by the CMTS at the end of the link drops. The CMTSwill then attempt to instruct the cable modem to increase the outputlevel to restore the desired input level for the CMTS. This may resultin either of two scenarios. First, the cable modem may not be able tofurther increase output level and the CMTS will list the cable modem asa problem unit that is not able to attain the desired input level to theCMTS. This does not mean that the CMTS can no longer receive signalsfrom the cable modem, as the CMTS has a wide input range to acceptsignals. Hence, the reverse path still generally functions whereas itwould have been severely impaired had the clipping problem not beenresolved. Second, the cable modem may have more headroom, in which casethe CMTS will instruct it to increase its output level and restore theCMTS input level to the desired value. As a consequence, the reverselaser will be driven into clipping again and the ONU microcontrollerwill further increase the RF attenuation. This cycle will continue untilthe cable modem has reached its maximum output capability and then thesystem is back to the first scenario.

The system shown in FIG. 5 provides protection from clipping by ONUs,and also causes the CMTS to be aware of problem modems or ONUs. As waspreviously noted, the root cause of the problem was that the loss fromONU to CMTS was too large, due for example to a bad fiber connection inthe optical network from ONU to the receiver. This problem is signaled,and eventually will be fixed. When the problem is fixed however, theCMTS input level increases beyond the preferred CMTS input level andthen the CMTS will direct the cable modem to reduce output level. If theONU is not at the nominal attenuation value and notices that the actualmodulation index is at or below the nominal level then this can berecognized as different from the previous “new value” for ONUs that hadbeen over-driven that was deliberately set above the nominal modulationindex. This implies that the problem in the system has been fixed andthe microcontroller can reduce the attenuation down to the nominalvalue, gradually or in one step. Thus, this technique automaticallyrecovers from the state where it protects the ONU from clipping withincreased attenuation to nominal attenuation once the system has beenfixed.

As previously indicated, an ONU takes time to turn on after a burst hasbeen detected. For example, the RFoG specification indicates that theturn-on time of an ONU should be between 100 ns thru 1000 ns (i.e. 1μs). A turn-on time that is too fast undesirably creates a very high lowfrequency noise, which decreases as frequency increases. Unfortunately,because this noise extends to around 50 MHz or beyond, most of thecurrently deployable upstream signals are propagated within thefrequency range that is affected by noise due to an abrupt turn-on time.Exacerbating the signal degradation is the fact that the noise is spiky,in that the instantaneous noise burst could be much higher than what iscommonly seen on a spectrum analyzer with moderate video bandwidth.

FIG. 6 generally illustrates an ONU upstream architecture 400 where anRF detector 410 detects whether an RF signal is present at its input420. If a signal is detected, the RF detector 410 passes the signalthrough to an amplifier 450 and also signals a laser bias control module430 to turn on at time t0 a laser 440, which has a turn-on time 460. Theamplifier 450 amplifies the RF signal that is passed through from the RFdetector circuit 410. The amplified signal drives the laser 440. Thelaser's output is propagated from the ONU on a fiber 470. Forsimplicity, the downstream ONU receiver architecture is not shown inFIG. 6. The turn-on time 460 of the laser has a profound effect on thespectrum produced by the turn-on event.

FIGS. 7 and 8 show estimated spectra for a rise time of 100 ns and 1 μs,respectively, for a typical signal at 40 MHz. For a short rise time, thenoise due to the ONU turn-on is of the same order of magnitude as theintended signal. With a slower laser turn on this effect can bemitigated.

If there is just one ONU on at any given point in time, the effect oflow frequency noise due to ONU turn-on is negligible, because the DOCSISload is inset after the laser has fully turned on. However, when thereare multiple ONUs that can turn on at any given time, then the noise isoften not negligible. If there was a first ONU on and a second ONU turnson while the first one is transmitting data, then the spikes in highnoise, described above, are present across a wide range of the frequencyspectrum of the upstream signal. Depending upon the relative RF levelsof the signals and the magnitude of the noise spikes, the signal mayexperience pre- or even post-forward error correction (FEC) errors, whenmeasured at the CMTS for example. The potential for debilitating noisebecomes more and more pronounced as the numbers of ONUs that can turn onincreases, as is likely to happen as architectures migrate to the DOCSIS3.1 standard. While this problem has always existed, it only becomesapparent, as a residual error floor, when the OBI and its induced errorsare eliminated.

An additional impairment is caused by the application of the RF signalbefore the laser has fully turned on and has stabilized. Specifically,an impairment can occur for example if the laser turn-on time is slowerthan the DOCSIS Preamble which may be applied before the laser hasreached steady state. Typically, the DOCSIS Preamble is sent as a QPSKsignal and can often be 6 to 10 dB higher than the regular RF signalthat follows, depending upon signal conditions. In such an instance, thelaser will be over-driven while still in a low power state andexperience very large clipping events that may cause spikes in noisethroughout the RF spectrum of the upstream signal, and thus hide othersignals that may exist at the same time. As previously indicated, whilethis effect has always occurred, it only becomes observable with theelimination of the OBI, and its attendant OBI-induced errors.

FIG. 9 shows a bias, around which a laser is modulated with a sine wavesignal. During the time that the laser bias is insufficient, the outputsignal is clipped. For slower laser turn-on, the duration of theclipping is increased. While it may be desirable to reduce the lowfrequency RF spikes that occur across the upstream frequency spectrum byhaving a slower turn-on time, the increase in clipping described abovemay counteract the benefit of the slow turn-on time. Disclosed are noveltechniques that permit a slow turn-on time while avoiding clippingartifacts.

Referring to FIG. 10, a novel ONU upstream architecture 500 includes anRF detector 510 that detects whether an RF signal is present at itsinput 520. If a signal is detected, the RF detector 510 passes thesignal through to an amplifier 550 and also signals a laser bias controlmodule 530 to turn on at time t0 a laser 540, which has a turn-on time560. The laser bias control module 530 preferably modulates the bias ofthe laser 540 to achieve a full turn-on of the laser 540 over a turn-ontime 560 that is preferably as slow as possible, e.g. the slowestturn-on time allowed by the RFoG standard, or in some embodiment evenlonger. In some embodiments, the turn-on time of the laser 540 could beup to 500 ns, or longer. This may greatly reduce the low frequencynoise. The turn-on time for the laser may be linear, as shown in FIG.10, or may implement a transition along any other desired curve, such asa polynomial curve, an exponential curve, a logarithmic curve, or anyother desired response.

The amplifier 550 amplifies the RF signal that is passed through fromthe RF detector circuit 510. The amplified signal drives the laser 540.Preferably, when amplifying the RF signal from the RF detector 510, thelaser bias control module 530 includes a circuit that modulates theamplifier gain to be proportional to the laser bias. This effectivelysets the gain of the amplifier 550 to be proportional to the laserturn-on 560, and thereby reducing or even preventing over shoot andclipping by the laser 540. The laser's output is then propagated fromthe ONU on a fiber 570.

FIG. 11 shows the output of the laser 540 when using the system of FIG.10. As seen in this figure, when using an RF gain factor proportional tothe laser bias, the clipping no longer occurs. However, the variation inRF level during the laser turn-on may potentially cause an issue in theburst receiver that may expect a near constant RF level during the laserturn-on. To mitigate this, in some embodiments, the amplifier bias maybe modulated to delay the RF signal to the laser, relative to theturn-on time of the laser 540, and may also apply a faster time constantthan the optical power turn on. This embodiment is illustrated in FIG.12.

FIG. 13 shows an implementation of an ONU that includes a delay in theRF signal to the laser, relative to the turn-on time of the laser, andalso applies a faster time constant than the optical power turn-on.Specifically, a novel ONU upstream architecture 600 includes an RFdetector 610 that detects whether an RF signal is present at its input620. If a signal is detected, the RF detector 610 passes the signalthrough to an amplifier 650 and also signals a laser/amplifier biascontrol module 630 to turn on at time t0 a laser 640, which has aturn-on time 560. The laser/amplifier bias control module 630 preferablymodulates the bias of the laser 640 to achieve a full turn-on of thelaser 640 over a turn-on time 660 that is preferably as slow aspossible, e.g. the slowest turn-on time allowed by the RFoG standard, orin some embodiment even longer. In some embodiments, the turn-on time ofthe laser 640 could be up to 500 ns, or longer. This may greatly reducethe low frequency noise. The turn-on time for the laser may be linear,as shown in FIG. 13, or may implement a transition along any otherdesired curve, such as a polynomial curve, an exponential curve, alogarithmic curve, or any other desired response.

The amplifier 650 amplifies the RF signal that is passed through fromthe RF detector circuit 610. The amplified signal drives the laser 640.Preferably, when amplifying the RF signal from the RF detector 610, thelaser/amplifier bias control module 630 includes a circuit thatmodulates the amplifier gain to be proportional to the laser bias, butwith a delay 680 relative to the time t0 that the laser 640 begins toturn on. Preferably, the rise time of the amplifier gain is faster thanthe rise time of the laser turn-on. In some embodiments, thelaser/amplifier bias control module 630 simply switches on the RF gain,i.e. the rise time is as short as the amplifier allows. The laser'soutput is then propagated from the ONU on a fiber 670.

This ONU shown in FIG. 13 effectively sets the gain of the amplifier 650to be proportional to the laser turn-on 560, and thereby reducing oreven preventing over shoot and clipping by the laser 640, while at thesame time mitigating problems caused by a receiver expecting anear-constant RF level during the time that the laser turns on. Theability to simultaneously reduce the laser turn-on time and to providean RF gain to the laser in proportion to the laser turn-on time, butdelayed with respect to the laser turn-on time is a feature that hasgreat potential in all applications, and without loss of generalitythese techniques may be used for any analog application such as DOCISIS3.0 or 3.1.

Either (or both) of the architectures shown in FIGS. 10 and 14 may beused together with the architecture shown in FIG. 2 so as to furtherimprove speed and stability of HFC systems. These may further be usedtogether with the long term clipping reduction discussed in the previousdisclosure to reduce the effects of both long term and short termclipping in the system.

Burst Detection

As indicated earlier, upstream transmissions typically operate inburst-mode (BM), where ONUs power up a transmitter, e.g. a laser, onlyduring time intervals when information is to be transmitted along theupstream path. A burst-mode system generally provides a lower noiseenvironment and thus enables better SNR, and in the case the transmitteris an optical device, the use of burst-mode tends to reduce Optical BeatInterference (OBI). Thus, in some preferred embodiments of the opticalcombiner system previously disclosed in this specification, where OBI isto be suppressed, such optical combiners are preferably operated inburst mode.

Also as indicated earlier, RFoG architectures that use burst-mode detectthe RF level in the ONU, powering the ONU's laser when an RF signal isdetected and powering down the laser when the RF signal is not present.This procedure is referred to as “RF detection.” In an optical combiner,the optical light inputs coming from the ONUs are all detected and thedetector outputs are collected. If RF detection is used with an opticalcombiner, an RF comparator would be applied to the output of thecombined RF output. If the RF level output of the combined RF detectorswere higher than the applied comparator, then the optical laser in theoptical combiner would be activated.

However, such detection may be fraught with difficulties because the RFlevel input could be very small. For instance, a very small slice of aD3.1 signal could be produced by any single ONU, hence the modulationindex of the ONU would be low, resulting in a low RF level at theoptical combiner. Also, optical input power to the optical combiner froma given ONU could be low; with an optical input range spanning up to 12dB, the RF level after detection could vary by 24 dB. As a result, theRF level from a photodiode could still be so low that this RF level thatis to be detected would be lower than the comparator, even if the RFlevel were high relative to the Optical Modulation Index of the ONUlaser that generated the RF signal. In ONU embodiments, the RF levelcould be turned on after the optical output is turned on, or while theoptical output is being turned on, such that the detection of an RFlevel at the disclosed optical combiner would be delayed. Furthermorethe detection could also be slow, because it depends upon the comparatorcircuit.

An alternative to using burst detection on the cascaded optical combinerunits disclosed in the present application would be to keep the upstreamlight transmission on all the time, irrespective of whether signals areprovided to the optical combiner or not, i.e. an “always on opticalcombiner”. Though this would ensures that the optical combinertransparently relays information upstream, however this would result ina constant light input at all the ports at an upstream optical combinerdevice or multiple port receiver. The total light input at the portsthus could lead to a summation of shot noise from all the ports,degrading the SNR performance of the total system. For this reason, inpreferred embodiments, the optical combiner unit transmits upstreamlight only when an RF signal has been received and is to be sent out.

Disclosed herein is a novel method of burst detection that is fast,simple, stable and robust thus enabling multiple new architectures.Specifically, broadly stated, the disclosed optical combiner system maymonitor the optical current of each photo diode as well as the sumcurrent of all photodiodes. If any one of the photo diodes registers aphoto current, or alternatively a current above a certain minimum value,the retransmitting laser is automatically turned on. The photodiodecurrent generation is instantaneous and beneficially is a DC value thatis easier to compare. As speeds of the interconnecting networks increaseover time, such optical detection circuits will become more useful.

Such an Optical Burst Mode (OBM) detector promotes reliability and mayhave the following advantages: (1) in the case of multiple daisy chainedoptical combiners as disclosed in the present application, substantialreduction in the additive shot noise is achieved relative to an “alwayson” solution; (2) in the case of DOCSIS 3.1 transmission, individualsignal transmissions with very low RF levels per ONU may be detected andretransmitted; and (3) in the case of varying optical input levels dueto different optical lengths between the ONUs and the disclosed activeoptical combiner, or varying optical lengths between multiple daisychained such active optical combiners, reliable burst mode operation maystill be achieved.

Furthermore, the disclosed novel burst detection also enables detectionof light at the input immediately at the start of a burst at the opticalcombiner input. Conversely, where there is no light at the input, oralternatively no light for a certain period of time, the ancillary RFamplifiers in the disclosed active optical combiner may be powered down,thus reducing the power dissipation of the disclosed active opticalcombiner. When light appears at the input of the disclosed activeoptical combiner, the amplifiers can be powered on again within the timeallowed; for instance in an RFoG system up to one microsecond is allowedto establish an optical link from the moment that the RF input isdetected and the system has started to turn on. Because RF amplifierstake a finite time to turn on and establish amplification; earlydetection of a burst is important to provide enough time to establishnormal operation. Such power cycling could reduce power dissipation byas much as ten times, thus drastically improving the criticalinfrastructure metrics. Thus, for example in the event of a poweroutage, the optical combiner can conserve the power required by not onlyusing optical burst operation, but also RF circuitry burst operation andextending a battery's life, if available.

Implementation of an optical power detection circuit capable of coveringa wide range of optical input power, in an architecture having multipledetectors is not trivial. Given the large number of detectors present,combined with a wide optical input power range, the amount and range ofphotocurrent that needs to be reliably detected is considerable. Simplymeasuring the voltage drop across a resistor in the detector biasnetwork is difficult; at low input power on a single detector, a smallvoltage drop can be reliably detected only if the value of a resistor,across which is a voltage drop equal to the photodetector bias, isrelatively high. However, increasing the value of such a resistor is notdesirable because this leads to an increased voltage drop when highdetector currents are present at multiple detectors; the detector biaswould become a strong function of the optical light present at thedetectors. In some embodiments, the detector bias is held constantbecause detector responsivity depends on detector bias; thus a varyingthe detector bias could lead to a variation in the gain of the system.Even a resistance value as low as a typical transmission line impedance,such as 75 Ohms, can be problematic when a large number of detectors areactive, and for instance 100 mA of detector current flows in themultiple detector system, leading to an excessive drop in detector bias.

Disclosed is a method to detect optical light over a wide input powerrange while retaining a constant bias on the detectors present in thetransmission line receiver. In order to accomplish this, a combinationof both an RF amplifier and a trans-impedance amplifier are used withthe multiple detector structure. In some embodiments, thetrans-impedance amplifier is connected to a high-pass structure in frontof the RF amplifier such that for low frequencies the trans-impedanceamplifier has a very low impedance connection (less than thetransmission line impedance) to the detector bias.

Referring to FIG. 14, which shows an example of a transmission linereceiver structure 700, a photo-detector may accurately modeled up tofairly high frequencies (˜1 GHz) by a capacitance in parallel with acurrent source for reasonable input power levels (>1 uW). Thus, in thisfigure, each of the circuit elements 710 would be a model of aphotodetector. Conventional receiver designs use a trans-impedanceamplifier or match the detector to as high an impedance as possible, soas to convert the current source signal to an RF signal with the bestpossible noise performance. Such approaches are limited by the detectorcapacitance such that an increase in the number of detectors or detectorarea leads to a loss of detector performance, and therefore a largenumber of detectors (e.g. 32) cannot reasonably be expected to workwell. This implies that multiple amplifiers are needed to receive alarge number of fibers.

A transmission line with impedance Z can be modeled by a ladder networkof inductors and capacitors with L/C=Ẑ2, which works well forfrequencies under the resonance frequency of L and C. Practical detectorcapacitance values are on the order of 0.6 pF, such that a 75 Ohmtransmission line would require L=3.4 nH. The resonance frequency iswell over 1 GHz such that, for up to 1 GHz, a transmission line with anarbitrary number of detectors compensated with 3.4 nH inductors wouldsimulate a 75 Ohm transmission line. The 3.4 nH can also be distributedaround the detectors as 2×1.7 nH, leading to a design as shown in FIG.14.

As indicated above, each current source/capacitor combination 710represents a detector. FIG. 14 shows a number of these in series,separated by respective transmission line sections 720 (100 psec or onthe order of 1 cm on board) having 75 Ohm impedance. The detectors arematched with 1.7 nH inductors 730. A 75 Ohm resistor 740 terminates theinput of the transmission line. The output 750 of the transmission linefeeds a low noise 75 Ohm RF amplifier (not shown). It should beunderstood that, although FIG. 14 shows six detectors, there is no limiton the number of detectors that can be combined by concatenating thesesections, and up to the LC resonance frequency there is negligibleimpact on the attainable bandwidth for a large number of detectors. Inpractice the 1.7 nH inductors could be implemented in the PCB layout asnarrower line sections, and a balanced transmission line with 100 Ohmdifferential impedance may be used to slightly improve noise figure.

As shown in FIG. 14, each current source/capacitor combination 710represents a photo detector, where the current source is the detectedcurrent in the detector; and the capacitor represents the parasiticcapacitance of the detector. Multiple detectors are connected withsections of transmission line (such as C2) and matching inductors (suchas L2 and L2). The matching inductors are chosen such that the parasiticcapacitance of the photo detectors is matched to the transmission lineimpedance (typically 75 Ohm). Thus multiple detectors can be combined,such that the detector currents are provided to the transmission lineand propagate both to the output 750 and to the termination resistor 740at the other end of the transmission line structure. The transmissionline structure bandwidth is limited only by the inductive matching ofthe photo-diode capacitance and can be very large, exceeding 1 GHz. Theoutput 750 is connected to an RF amplifier matched to the transmissionline impedance, which amplifies the signals output from the transmissionline structure. Note that use of a trans-impedance amplifier that is notmatched to the transmission line structure would cause a very largereflection of the output signals back into the transmission linestructure; a trans-impedance amplifier is not a preferable means toamplify the output from a transmission line receiver.

Typically the photo detectors need to be biased, for instance with 5 V.In order to decouple the bias voltage from the amplifier, a decouplingcapacitor may typically be used. The bias can then be provided via aninductor in a bias-tee arrangement as shown in FIG. 15, for example. Thesignal from the transmission line 760 is provided to an amplifier (notshown) via a capacitor (770) that passes high frequency signals, andbias from a voltage source 775 is provided to the transmission line viaan inductor 780 that passes low frequency signals. The terminationresistor 740 at the other end of the transmission line is thuscapacitively decoupled to permit a DC bias. The current through voltagesource 775 can be measured to determine photocurrent; the voltage source775 could be constructed as a trans-impedance amplifier providing aconstant voltage and an output proportional to the current provided.However, in implementations, the inductor 780 needs to be chosen with avalue large enough that it does not affect the low frequency response ofthe amplifier. As a consequence, there may be a delay in the response ofthe current in the inductor 780 to a change in photo detector current,and this ten ds to cause a delay in the detection of photocurrent.

FIG. 16 shows an implementation 800 that uses both ends of thetransmission line receiver structure to alleviate such a delay. Theresistor R1 in FIG. 16 is the termination resistor 740 shown in FIG. 14,and the inductor L1 is the inductor 780 in FIG. 15. The voltage source810 provides bias both to the termination resistor 740 and the inductor780. The current in resistor 740 responds instantly to a photocurrentsuch that a fast detection of photocurrent is enabled. The inductor 780can support large photocurrents without a significant voltage drop suchthat large photo currents can be supported without a significant drop inbias to the photo detectors. A capacitance 815 can be placed adjacent tothe voltage source 810; for an ideal voltage source it may not carry anycurrent because the voltage is constant. However at RF frequencies itcan be difficult to realize a perfect voltage source, hence thecapacitor 815 provides a low impedance to ground such that RF currentsin the termination resistor 740 do not cause modulation of the voltageat the voltage source 810.

In order to realize an efficient detection circuit for the current involtage source 810, the voltage source 810 is preferably implemented asa trans-impedance amplifier. A trans-impedance amplifier is a basicelectronic circuit that holds a node between two current paths at aconstant voltage and has an output that changes its output voltage inproportion to the current provided at that node. Thus, externally thetrans-impedance amplifier looks like a voltage source to that node, butthere is an additional output that represents the current provided. Thisoutput may then be used to drive a decision circuit to decide if aphoto-current flows or not. Due to the fact that the trans-impedanceamplifier is realized with a practical transistor circuit, it does nothave infinite bandwidth, which means that it is not able to hold thenode voltage constant for very high frequencies and for that reason thecapacitor 815 may be added in some embodiments.

It should be understood that in some embodiments, the LC bias networkprior to the amplifier (capacitor 770 and inductor 780) may be replacedby more complex circuits, or even with diplex filters—provided that thenetwork provides a low-loss, high-frequency path from the transmissionline detector to the amplifier, and a low-loss (low impedance) path atlow frequency from the voltage source (trans-impedance amplifier) to thetransmission line detector bias. It should also be noted that thetrans-impedance amplifier may be implemented such that the outputvoltage first changes linearly as a function of photo-current, but thensaturates at a photo-current that is sufficiently high.

In other implementations, a photocurrent detection circuit may beapplied to each individual photo detector; optionally one electrode of aphoto detector (for instance cathode) may be connected to an RF circuitand the other electrode (for instance anode) may be connected to anoptical power detection circuit. This increases complexity, as adetection circuit is required per detector. Also, some embodiments mayoptionally use a trans-impedance amplifier per detector.

With an optical burst mode detection circuit, for instance of the typedescribed above, the bias of a laser or the bias or gain of an amplifiermay be controlled. FIG. 17 shows a multiple-detector receiver 820 thatproduces an output 825 signaling that power has been detected from anyone of multiple inputs 830. This detection can be based on a detectionmethod as described in the previous section or on multiple detectorcircuits that are monitoring individual detectors 835. When opticalinput has been detected at time t0 then the laser bias is turned on witha controlled rise time t_on_1 and the active combiner can re-transmitsignals present at the inputs.

The optical burst mode detection can further be used to control theamplifier bias as shown in FIG. 18; when optical power is detected at t0the amplifiers are immediately turned on. The laser turns on more slowlysuch that the amplifiers are settled by the time that the optical poweris on. Optionally this scheme may be expanded by a third control signal850 that controls the amplifier gain, as shown in FIG. 19.

Optical Modulation Index and Self Calibration

For implementations that permit operation of all upstream inputs of theactive splitter simultaneously the total amount of photocurrent on thedetectors following the upstream inputs can be high. The impedance ofthe bias circuit and, as discussed, of the aforementioned filteringmeans in the detector output path must be low.

In an existing RFoG system, the CMTS controls the output level of thecable modems' communications with ONUS that are transmitting RF signalsto a head end such that a desired input level to the CMTS is obtained.This implies that the output level from a receiver preceding the CMTS isadjusted to a known level. If this receiver is of a type that has aknown amount of gain such that an output level corresponds to a knownoptical modulation index, then this implies that the optical modulationindex of channels provided to the CMTS is known—given the RF signallevel to which the CMTS adjusts the channel. This requires a calibratedreceiver that adjusts its gain as a function of the optical input level(2 dB gain increase for every dB reduction in optical input level) suchthat this fixed relation between RF output level and optical input levelis maintained. The modulation index into the receiver is the modulationindex of the upstream laser in the active splitter connected to thatreceiver; thus the CMTS implicitly controls the modulation index of thatactive splitter output.

The gain of the active splitter should preferably be set such that anoutput modulation index from that active splitter has a known relationto an input modulation index at one or more of the photo detectorsreceiving upstream signals from active splitters or ONUs furtherdownstream. This requires knowledge of the photocurrents at these photodetectors, and preferably the active splitter can monitor the photocurrent of each upstream link by using one detector per upstream link asin a transmission line detector, for instance. Since some systems mayoperate in burst mode, these photo currents are not always available.However, in a DOCSIS system all ONUs are polled repeatedly to obtain anacknowledgement signal with an interval up to five minutes. This impliesthat upstream active splitters are re-transmitting the information, andall active splitters in such a system have each one of the upstreaminputs active at least once every five minutes. The active splitter canthus record the burst levels and build a map of optical input levels toinput ports. Using this information, the active splitter can set aninternal gain level such that the upstream modulation index ismaximized, but will not clip so long as the input signals to the activesplitter are not clipping. Whereas the fiber length from head end tofirst active splitter is generally long, those fiber lengths betweenactive splitters and those fiber lengths from active splitters to ONUsare generally short, and have little enough loss that the optical inputpower values to the different upstream input ports are close, and theoptimal gain setting is similar for all ports. As a consequence, theoptimal gain setting in the active splitter is almost the same for allinput ports and the compromise in SNR from assuming a worst case reverselaser modulation index from a signal on any of the input ports is small.

As noted earlier, one embodiment could use the high and low opticaloutput power setting for the reverse laser, instead of switching thelaser between a high output power for burst transmission and an offstate in between. Not only does this embodiment provide continuousinformation to active splitters about the link loss to the ONU, it alsoimproves laser operation. When a laser powers on, the transient leads toa brief transition where laser distortion is high and RF input signalscan be clipped. If a laser is held at a low power level instead of beingin the off state before being turned on to a higher power level, thenthis transient is near absent and distortions and clipping are reduced.In case the laser is held at a high output power continuously, thesetransients and distortions are absent. The active splitter architecturepermits operating the ONUs in any of these three modes and an optimumcan be selected for system operation.

Whereas the upstream input power levels to detectors on an activesplitter are typically similar, in some instances they may differ due todifferences in connector loss or fiber loss. Preferably, all opticalinputs would have the same level or have the same RF level following thedetector for an equivalent channel load. Since the active splitter canmonitor the power level at each detector and map those optical inputlevels, it can compute adjustments to optical input power level or inmodulation index of those inputs that would be required to equalize theRF levels following the detectors of each input. The active splitter cancommunicate those preferred settings for output power level or gain forthe reverse transmitters downstream that are connected to the inputs.The communication signals can be modulated onto a laser injected intothe downstream signals or onto pump laser currents in EDFAs amplifyingdownstream signals. The modulation can be selected to be small enough,and in such a frequency band, that the communication signals do notinterfere with the downstream payload.

Preferably, not only active splitters receive and interpret thesecommunication signals, but also downstream ONU units receive andinterpret the signals. This would permit essentially perfect alignmentof the optical transmission level and RF gain of all units in an activesplitter system. Given the presence of an upstream laser, and theability of all components in an active splitter system to receive anupstream signal, all components in an active splitter system are capableof upstream communication with the addition of a simple tone modulationor other scheme. Thus, bidirectional communication is enabled, andactive splitters and the head end can communicate with each other,self-discover the system, and setup optimal gain and optical levels.

One objective of the active splitter architecture is to provide accurateRF levels to the CMTS that represent an optical modulation index. Doingso is not trivial, and requires a specific self-calibration procedure(later described) that is expected to result in accurate modulationindex correlation to active splitter head end receiver output RF level.The receiver is either a CMTS plug-in or is connected directly to theCMTS without unknown RF loss contributions in between (in case a tap isneeded for other services than the CMTS, the tap can be integrated inthe receiver to avoid external RF losses). As a consequence, themodulation index of the active splitter re-transmitter units is setprecisely.

In case bidirectional communication is not available then the ONU outputpower level cannot be adjusted by the active splitter and the modulationindex of the ONU will still have some uncertainty since the optical lossbetween ONU and the active splitter/receiver can vary; a +/−1 dB lossvariation from ONU to active splitter would result in a +/−2 dBtolerance in RF level, thus a dynamic window will at least have toaccommodate that variance and headroom for other tolerances and CMTSsetup accuracy. This should be readily available for bandwidths up to200 MHz such that even without the active splitter controlling the ONU,output power acceptable system performance can be obtained With theaforementioned bidirectional control additional system headroom can beobtained.

When 1200 MHz return bandwidth is used, such that ONUs are assigned 200MHz widths of spectrum, the ONUs can all be operated a few dB belowtheir clip point, i.e. just enough to cover the uncertainty in the lossfrom the ONU to the active splitter to avoid clipping of the ONUs. Thisoptimizes the performance of the critical link from the ONU to theactive splitter, so that 0 dBm ONUs are sufficient. In this type ofoperation, an arbitrary choice can be made for the number of ONUsoperating with such a 200 MHz band, for instance up to six ONUs. This inturn would cause clipping in the active splitter transmitter, thus for1200 MHz operation the gain of the active splitter receivers followingthe ONUs can be reduced by 8 dB, such that when six ONUs aretransmitting 200 MHz of signal bandwidth, the active splitter reversetransmitter is operated just below clipping. This method of operationmaximizes SNR and eliminates uncertainty; the impact of variation of theONU to active splitter link is minimized, and the active splitter linksare operated with a precise modulation index as with lower bandwidth RFreturn systems. The required dynamic window is reduced to tolerances inCMTS level setting and active splitter output level calibration tomodulation index-comparable to a forward transmitter.

Analysis of attainable SNR by using the system just described for 1200MHz operation with a maximum load of 200 MHz per ONU, results in a 5 dBimprovement in the SNR attainable at 1200 MHz. This results in about 20%more throughput capacity in the system. With 1200 MHz of bandwidth, thetotal upstream data rate could be as high as 10 Gbs.

In case the system is initially set up so that the active splitter unitsexpect a 1200 MHz return spectrum (instead of for instance 200 MHz) witha maximum of 200 MHz per ONU, then a penalty of around 7 dB occurs interms of peak NPR performance. Therefore, the mode of operationpreferably can be switched between normal operation, where a single ONUcan occupy the entire spectrum, and high bandwidth operation where asingle ONU can be assigned a limited amount of spectrum at any time andthe active splitter reverse transmitters support the entire spectrum atonce.

The proposed architecture has multiple re-transmission links that arepreferably operated at the best possible modulation index on theassumption of perfect alignment of the NPR (Noise Power Ratio) curves ofthose links. As noted earlier, the alignment of the re-transmission inthe active splitter return links is critical to obtain the best possibleperformance (every dB of misalignment directly results in a reduction ofavailable SNR) hence a calibration technique is needed to set and holdthe correct alignment of transmitter gain factors.

In order to provide such calibration, the active splitter returntransmitter gain will be set accurately such that for a given detectorcurrent of the active splitter receiver diodes, the modulation index ofthe transmitter is equal to the modulation index input to the detector.This only requires knowledge of the detector current; the actual opticalinput power to the detector and the detector responsivity areirrelevant. In order to accomplish this, means are implemented at eachdetector to measure detector current such that an appropriate gain canbe set for the return transmitter.

The gain may be set individually for each detector, but since multipledetectors can be receiving signals at the same time, this would requirea controllable attenuator for every detector (32 detectors are in atypical active splitter unit). Preferably, a single attenuator is usedfor all detectors. This is achieved using variable output transmittersin the active splitter units, communicating to an upstream activesplitter or variable output transmitters in ONUS communicating to anupstream active splitter. Outlined below is a method to set the outputlevel of each of the reverse transmitters such that each transmitterprovides the same photocurrent on the detector to which it is coupled.During normal operation, the active splitter receiver monitors thedetector currents during bursts to enable issuance of a warning in casean optical link degrades or is lost.

For a 1310 nm reverse link from the active splitter to an upstreamactive splitter, the reverse laser power typically needs to becontrolled from either 3-10 dBm or 6-10 dBm, depending on the design ofthe active splitter receiver. For a 1610 nm reverse link, these figuresare typically 3-7 dBm or 6-7 dBm, respectively. These controls ensurethat the power received at the end of a 25 km link, with some WDM loss,is at least 0 dBm. It should be understood that the numbers given areexamples. The active splitter can transmit information in the forwarddirection through pump modulation of the EDFA or injection of a signalinto the forward path. The latter is more expensive; the former resultsin a lower data rate, as only a minimal pump fluctuation can be allowedwithout affecting the forward path. A low data rate is sufficient, andcan be read by a simple receiver—for instance a remote controllerreceiver operating in the kHz range coupled to a low cost processor. Itshould be understood that the downstream transmit function is onlyrequired in upstream active splitter units unless ONUs are beingcontrolled as well. In the figures shown, that would be one out of 33active splitter units in the system.

In a self-calibration run, the upstream active splitter unit transmits acommand downstream to active splitter units to initiateself-calibration. Subsequently the downstream units randomly turn theirtransmitters on and off at full power with a low duty cycle, such thatin nearly all cases at most one of the downstream units is on. Theupstream active splitter reports information downstream as to which portis on, and what detector current it has obtained from that unit. Thedownstream units record that information in non-volatile memory. Afterall ports have been on at least once, or a time out has occurred (forinstance if one or more ports are not connected), the upstream activesplitter unit determines which downstream active splitter produces thesmallest detector current. Next, the upstream active splitter computeshow the upstream powers of each of the downstream units should be set,such that all detector currents are the same and fall within a specifiedrange. That range can for instance correspond to 0-3 dBm (or 6 dBm)input power at the detectors. It should be understood that this can beaccomplished by setting a photodetector current, and does not requiremeasurement of an exact optical input power.

Generally, the active splitter upstream unit will set this power to thebest (or maximum) value that can be obtained to optimize the SNR of thelinks. The active splitter units will then all have a known outputpower, and their internal gain will accordingly be set to have acalibrated modulation index for a given input power and modulationindex. All links into an upstream active splitter may behaveidentically. The upstream active splitter unit may then take thedownstream units out of calibration mode.

In case an additional port is lit up on an upstream active splitterreceiver port, then the self-calibration algorithm can proceed withoutservice interruption of already connected active splitter units. This isachieved by activating self-calibration on the downstream activesplitter receiver that has just been activated. Its output will turn onand the upstream active splitter unit will then assign a port number tothe new, hitherto unused port and set a power to the new unit, and takeit out of calibration mode.

During normal operation, the upstream active splitter unit continues tomonitor receiver currents for the incoming upstream links. If there issignificant deviation, it may still issue a non-calibration modedownstream command to re-adjust power, and it can also signal plantissues upstream.

The active splitter units operated in the disclosed manner can alsobuild a map of connected active splitter units. Also, a map can becreated of upstream power from connected ONUs and statistics onindividual ONU operation and link loss can be collected, for instance tolocate chattering ONUs or poor ONU connections.

The head end transmitter can also send a command to downstream activesplitter units to initiate calibration or change a mode of operation(for instance from 200 MHz to 1200 MHz optimized operation). Any othertype of bidirectional EMS system monitoring can be envisioned for activesplitter units that can receive and transmit low data rate traffic. Itshould be understood that this does not require complex or costly HFCEMS systems; minor optical power fluctuations by either pump powervariation or low level signal injection in the downstream signal path,or reverse laser power variation in the upstream path, are sufficient todetect binary or kHz range (like remote control chips) modulated datapatterns. It should also be understood that the most expensiveoption—injection of a downstream optical signal—is only relevant at thehead end, or in the upstream path typically only relevant in 1 out of 33active splitter locations.

Another important consideration is that the CMTS should set up modemlevels correctly. In regular return or RFoG systems, there isconsiderable uncertainty in system levels due to RF components orapplied combiner networks. In the active splitter system, however, thereare no RF components in the link, the service group is aggregated in theoptical domain, and only one low gain, low performance, and low outputlevel receiver is required which is coupled directly to the CMTS returnport. In some embodiments, it may be desirable to produce a dedicatedactive splitter receiver with an accurately calibrated output level as afunction of input modulation index. Such a receiver has no need for awide input range; −3 to +3 (or 0 to +6) dBm is sufficient. The highinput level implies that the gain can be low. The absence of RFcombining following the receiver also means that the output level can below. Therefore, such a receiver should be obtainable in a high density,low power form factor. With such a receiver, little if any RF wiring maybe required in the head end, and the CMTS can accurately set reverselevels to obtain the correct optical modulation index. In some cases,there may be a need to connect other equipment than the CMTS to thereverse path. The receiver may use an auxiliary output to provide forthis functionality, rather than the main output with external RFsplitters. This eliminates any level uncertainty due to RF componentsbetween the receiver and the CMTS.

Embodiments

Some embodiments of the foregoing disclosure may encompass multiplecascaded active splitters that are configured to work with ONUs basedprimarily on optical input levels without requiring bidirectionalcommunication. Other embodiments may encompass multiple cascaded activesplitters that are configured to work with ONUs by using bidirectionalcommunication.

Some embodiments of the foregoing disclosure may include an activesplitter with multiple optical inputs, each providing an optical inputto one or more detectors that together output a combined signal to ahigh pass filter that presents a low impedance to the detectors andrejects all signals below an RF frequency band and passes all signalsabove an RF frequency band before presenting the combined signal to anamplifier and a re-transmitting laser.

Some embodiments of the foregoing disclosure may include an activesplitter with multiple optical inputs, each providing an optical inputto one or more detectors that together output a combined signal, wherethe active splitter has a bias circuit with a sufficiently low impedanceat low frequency such that all detectors can be illuminated at the sametime without a significant drop in bias to the detectors.

Some embodiments of the foregoing disclosure may include an activesplitter with a reverse laser where the reverse laser turns on when aphotocurrent at the active splitter input detectors is above athreshold, and where the slew rate when the laser turns on is limitedsuch that it does not create a transient having a spectrum thatinterferes with the upstream spectrum to be transmitted.

Some embodiments of the foregoing disclosure may include an RFoG activesplitter architecture where reverse lasers of the active splitter(s)and/or ONUs connected to the active splitter(s) are operated with acontinuous output. Some embodiments of the foregoing disclosure mayinclude an RFoG active splitter architecture where reverse lasers of theactive splitter(s) and/or ONUs connected to the active splitter(s) areoperated between a high and a low power mode such that the output poweris high during bursts of upstream transmission and is otherwise low inoutput. Some embodiments of the foregoing disclosure may include an RFoGactive splitter architecture where reverse lasers of the activesplitter(s) and/or ONUs connected to the active splitter(s) may beselectively set to either one of a continuous mode and a burst mode.

Some embodiments of the foregoing disclosure may include an RFoG ONUthat switches between a high and a low output power state where theoutput power is high during burst transmission of information and wherethe low output power state is above the laser threshold.

Some embodiments of the foregoing disclosure may include an RFoG systemthat measures detector currents at all inputs, building a table ofdetector currents during high and low (or no) input power to the opticalinputs and computes, based on that table, a gain value such that amodulation index of the reverse transmitting laser has a known relationto a modulation index at the optical inputs to the active splitter, suchthat the reverse transmitting laser has an optimal modulation index butclipping is prevented, even for the port with the highest optical input.In some embodiments of the foregoing disclosure, the optimal modulationindex of the reverse transmitter is nominally the same as that for theoptical inputs.

Some embodiments of the foregoing disclosure may include an RFoG ONUwith an RF signal detector that detects bursts of input signals andactivates a laser at a high power mode when a burst is detected andotherwise activates the laser at a low power mode, such as zero power.An electrical attenuator may precede the laser driver and may attenuatean RF input signal, such that in the low output power state the lasercannot be clipped by an RF input signal. The RF attenuation before thelaser may be reduced as the laser power increases from the low powerstate, such that the RF attenuation is rapidly removed to have minimalimpact on the burst but during the transition, the laser still is notclipped.

Some embodiments of the foregoing disclosure may include an RFoG ONUwith an RF signal detector that detects bursts of input signals andincludes an electrical attenuator that precedes the laser driver toattenuate the RF input signal, such that when no nominal input ispresent noise funneling by the ONU of weak noise signals into the ONU isprevented and RF attenuation is rapidly removed when a burst is detectedto have minimal impact on the burst.

Some embodiments of the foregoing disclosure may include an RFoG ONUthat can receive a downstream signal instructing it to adjust outputpower level, RF gain or both. In some embodiments, such an ONU canreceiver assigned port numbers and status monitoring requests. In someembodiments, such an ONU can transmit upstream information such asstatus, serial number, etc.

Some embodiments of the foregoing disclosure may include an activesplitter than can transmit a downstream signal with requests todownstream units to adjust optical power level, gain or to requeststatus information. Some embodiments may include an active splitter thatcan receive such downstream signals. Some embodiments may include anactive splitter that can transmit and/or receiver such signals in theupstream direction, as well.

Some embodiments of the foregoing disclosure may include an ONU with anRF detector, an attenuator, a bias circuit, and a microcontroller wherethe microcontroller estimates laser clipping based on measured RF powerlevels and tracks what fraction of the time the laser is clipping andincreases attenuation in case this fraction exceeds a threshold. Themicrocontroller may also adjust laser bias to prevent clipping. In someembodiments, the microcontroller brings attenuation to a nominal valuewhen RF power to the laser is at or below a nominal value. In someembodiments, changes in attenuation made by the microcontroller takeplace in discrete steps in time and magnitude.

In some embodiments of the foregoing disclosure the microcontroller mayset the attenuation to a high enough level to prevent clipping but lessthan needed to obtain a nominal modulation index.

Some embodiments of the foregoing disclosure may include a bidirectionalRF-over-fiber architecture with more than one re-transmission link inthe reverse direction, where detected signals from preceding links arecombined at each re-transmission link.

Some embodiments of the foregoing disclosure may include a calibratedreceiver at a head-end that provides a specific RF output level for aninput modulation index, with a gain control such that for differentoptical input levels, the RF output level for a given modulation indexis held constant. In some embodiments, a receiver may include twooutputs, at least one connected to a CMTS without any RF combining andsplitting networks.

Some embodiments of the foregoing disclosure may include an activesplitter with at least two gain settings, one gain setting optimized forONUs that can transmit the full reverse spectrum that the system cansupport, and one setting optimized for ONUs that can transmit an amountof spectrum less than the full spectrum that the system can support,where the active splitter combines inputs from multiple ONUs and cantransmit the full spectrum that the system can support.

Some embodiments of the foregoing disclosure may include an activesplitter having adjustable reverse transmission power and adjustablegain such that, for a given received upstream signal modulation index,the active splitter maintains a constant optical modulation indexirrespective of optical output power. In some embodiments, theretransmitted optical modulation index is the same as the receivedoptical modulation index. In some embodiments, the retransmitted opticalmodulation index is a predetermined fraction of the received opticalmodulation index, and the splitter enables an option to vary thatfraction.

Some embodiments of the foregoing disclosure may include an activesplitter that can receive and decode forward communication signals, e.g.an input-monitoring diode for an EDFA, or another monitoring diode.

Some embodiments of the foregoing disclosure may include an activesplitter that can transmit forward communication signals, with forinstance a forward laser, or by modulating the pump current of an EDFA.

Some embodiments of the foregoing disclosure may include an activesplitter that can receive and decode upstream communication signals,e.g. by monitoring upstream detector currents. Some embodiments of theforegoing disclosure may include an active splitter that can transmitupstream communication signals, e.g. by modulating the reverse laser.

Some embodiments of the foregoing disclosure may include a system withat least two active splitters where a first active splitter instructs asecond active splitter to adjust its reverse transmission power level.Some embodiments may use an algorithm to equalize and optimize thereverse transmit level of all downstream active splitters connected toan upstream active splitter. I some embodiments, the algorithm isexecuted automatically at start up such that downstream active splitters(and optionally ONUs) obtain an address and optionally report in theupstream direction the splitter's (or ONU's) serial number and status.In some embodiments, later activation of ports in the splitter leads toan automatic calibration of new ports without interrupting the serviceof existing ports, and with continuous monitoring of port health.

Some embodiments of the foregoing disclosure may include an activesplitter capable of upstream communication, and capable of receiving anddecoding upstream communications from another splitter.

In some embodiments, an active splitter may establish a map of thesystem in which it is included, and may report system status andtopology information to a head, end and may issue alarms if necessary.The map may include serial numbers of active splitters, and may includeserial numbers of connected ONUs. Some embodiments may create a systemmap automatically, and (i) may monitor ONU link input levels to activesplitters; (ii) may detect chattering or otherwise defective ONUs andoptionally instruct active splitter to shut down detectors of defectiveor chattering ONUs; and/or (iii) may monitor the status of the activesplitter that constructs the map. In some embodiments, the monitoringfunction is used to automatically trigger route redundancy by monitoringupstream traffic on a link, to determine if the link is intact, and ifthe link is found to be defective, switching downstream traffic to analternate upstream link. In some embodiments, upstream active splittersmonitor downstream active splitters by communicating with downstreamactive splitters.

Some embodiments of the foregoing disclosure may include a head end thatinstructs downstream active splitters to initiate a self-calibrationprocedure.

Some embodiments include a combiner that can monitor each of theupstream input ports and thus detect a loss of a link to such a port.The loss of an upstream link implies that the associated downstream linkhas been lost. Detection of a link can be used to initiate switchingover to a redundant fiber link, preferably following a different fiberroute.

In one or more examples, the functions described herein may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium and executed by a hardware-based processingunit. Computer-readable media may include computer-readable storagemedia, which corresponds to a tangible medium such as data storagemedia, or communication media including any medium that facilitatestransfer of a computer program from one place to another, e.g.,according to a communication protocol. In this manner, computer-readablemedia generally may correspond to (1) tangible computer-readable storagemedia which is non-transitory or (2) a communication medium such as asignal or carrier wave. Data storage media may be any available mediathat can be accessed by one or more computers or one or more processorsto retrieve instructions, code and/or data structures for implementationof the techniques described in this disclosure. A computer programproduct may include a computer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

1-17. (canceled)
 18. A Radio Frequency over Glass (RFoG) system havingat least one aggregating optical splitter/combiner connected via a fiberto one or more contributing optical splitter/combiners locateddownstream, comprising: one or more first fibers, where the at least oneaggregating optical splitter/combiner detects multiple optical inputsignals input to combiner ports, and re-transmits radio frequency (RF)modulated information from each of these ports into the one or morefirst fibers; one or more second fibers connected to a port of the atleast one aggregating optical splitter/combiner and at least one of theone or more contributing optical splitter/combiners, each of the one ormore contributing optical splitter/combiners configured for transmittingradio frequency (RF) modulated signals to the one or more second fibersconnected to a port of the at least one aggregating opticalsplitter/combine; and one or more third fibers connecting the one ormore contributing optical splitter/combiners to an optical networkingunit (ONU) such that radio frequency (RF) modulated information sentfrom the optical networking unit to at least one additional fiber isreceived by the one or more contributing optical splitter/combiners andre-transmitted by the one or more contributing opticalsplitter/combiners to the at least one aggregating opticalsplitter/combiner via the one or more second fibers.
 19. The system ofclaim 18, the one or more first fibers for transmitting radio frequency(RF) signals between the aggregating optical splitter/combiner and aheadend, the one or more second fibers for transmitting radio frequency(RF) signals between an aggregating combiner and the one or morecontributing optical splitter/combiners, and the one or more thirdfibers for transmitting radio frequency (RF) signals between an opticalnetwork unit (ONU) and the one or more contributing opticalsplitter/combiners.
 20. The system of claim 18, further comprising atransmitter and a multiplexer for relaying content between the firstactive combiner and the second active combiner, wherein: the at leastone aggregating optical splitter/combiner operates as a splitter in aforward direction and an active combiner in a reverse direction, the oneor more contributing optical splitter/combiners receive respectiveoptical signals from each of a plurality of subscribers, combine thereceived optical signals to create a combined electrical signal, andamplify the combined electrical signal, the transmitter receives theamplified combined electrical signal and converts it to a reverse pathoptical signal; and the multiplexer multiplexes the reverse path opticalsignal with a forward path optical signal.
 21. The system of claim 18wherein at least one of the one or more second fibers includes a signalfiber for transporting signals in the downstream and the upstream, andat least one of the one or more second fibers, different from the signalfiber, for transmission of at least one of 980 nm light or 1480 nm lightto the one or more contributing optical splitter/combiners providingpower to the one or more contributing optical splitters/combiners. 22.The system of claim 21, wherein the at least one aggregating opticalsplitter/combiner includes at least one of a 980 nm laser, a 1480 nmlaser, or a 850 nm laser, and adds SBS suppression to the at least onelaser by modulating a current of the at least one laser.
 23. The systemof claim 21, wherein an EDFA in the one or more contributing opticalsplitter/combiners is optically pumped via the at least one of the oneor more second fibers transmitting light downstream to the one or morecontributing optical splitter/combiners.
 24. The system of claim 23,wherein the EDFA is a passive optical device comprising erbium dopedfiber and receives the at least one of the 980 nm or 1480 nm lightgenerated by the at least one aggregating optical splitter/combiner asan optical pump
 25. The system of claim 18, wherein a single first fiberprovides both optical signals and at least one of 980 nm fiber power or1480 nm fiber power generated by the aggregating opticalsplitter/combiner to the one or more contributing opticalsplitter/combiners for both upstream and downstream processing by theone or more contributing optical splitter/combiners, the one or morecontributing optical splitter/combiners employing high power capablewave division multiplexer (WDM) optical passives in the upstreamcircuitry and passive downstream circuitry.
 26. The system of claim 18,wherein a single first fiber provides both optical signals and at leastone of 980 nm fiber power or 1480 nm fiber power generated by theaggregating optical splitter/combiner to the one or more contributingoptical splitter/combiners, wherein the fiber power is used for poweringupstream circuitry in the one or more contributing opticalsplitter/combiners and for optically pumping a downstream EDFA in theone or more contributing optical splitter/combiners.
 27. The system ofclaim 18, wherein a 1550 nm signaling transmitter is co-located in asame transmitter module with at least one 980 nm or 1480 nm pump laser.